How to convert passphrase into a 'WPA Shared Key' for a Linksys WAP54G v1.1 Access Point?

Details of the calculation  
For WPA-PSK encryption, the binary key is derived from the passphrase according to the following formula:     Key = PBKDF2(passphrase, ssid, 4096, 256)
 
The function PBKDF2 is a standardized method to derive a key from a passphrase. It is specified in RFC2898 with a clear explanation on how to compute it. The function needs an underlying pseudorandom function. In the case of WPA, the underlying function is HMAC-SHA1.
 SHA1 is a function that computes a 160-bit hash from an arbitrary amount of input data. It is clearly explained in RFC3174. HMAC is a standardized method to turn a cryptographic hash function into a keyed message authentication function. It is specified in RFC2104.  
 To summarize, the key derivation process involves iterating a HMAC-SHA1 function 4096 times, and then doing that again to produce more key bits. The amount of computation involved is equivalent to computing the SHA1 hash over 1 MByte of data. Perhaps that explains why the Javascript on this page is so slow.

Unless you're using the setup wizard or something, you should be able to enter the desired key directly, without having to convert it. The only part of the security menu with a passphrase converter is if WEP is chosen.

Where is this passphrase coming from?
i.e. what else has already been setup using that passphrase in a converter/generator?

Also, what version of firmware is loaded?  It should tell in the upper right corner of the http menus.
Firmware v3.04 is the newest available for WAP54G v1.1 according to
http://www.linksysbycisco.com/US/en/support/WAP54G/download

According to WPS54G's data sheet, it does not support WPA2, only WPA.
So try setting the router to WPA-Personal and use TKIP encryption rather than AES.

It's possible to capture TKIP'd packets and crack their encryption offline, ergo it's possible (with much work) to expose the data (I have not yet seen any claims of successfully cracking AES encryption), but the crackers cannot follow the vectors back to expose the original password, so your connection would still be secure... and every time the password changes (according to the renewal time) they would have to capture more packets and crack the encryption again.

IE8 cannot attach files on EE since the MS updates about 3 weeks ago, so here's a link to where the data sheet can currently be found:
http://www.linksysbycisco.com/US/en/products/WPS54G

You should be able to connect the print server to the router temporarily with a short network cable and access its setup menus like that. Connecting the print server directly to a computer's NIC may require a crossover cable.

Use the IP address that you assigned with the Setup Wizard. If you don't remember that IP, usually it will be on the diagnostic page that prints when you press the Reset button. Otherwise run the setup wizard using the setup CD. Follow the prompts until you reach the Wireless-G PrintServer Setup screen. Note the IP address, then back out until you're at the Welcome screen and click Exit.

If there's no way to connect it to a LAN port on a switch/the router, and you don't have a crossover cable, reset it to defaults by unplugging it, hold in the reset button, then plug the power back in on the printserver. Continue holding in the reset button at least 10 seconds (usually the LEDs on the front will flash when it resets)... without the power being dropped first, pressing the reset button will just print a diagnostics page as noted above.

It sounds like you understand it fine. The wireless security/encryption passphrase controls only the wireless connection, not access to the setup menu per se... so if the printserver is connected with a network cable the wireless security passphrase doesn't matter.

Personally, our printserver here is connected to the LAN with a cable even though it's possible to connect it wirelessly... I find it does print a little faster on most jobs like that (i.e. except from laptops connected to the LAN wirelessly).  But if you don't want to run cable to it permanently, there's nothing inherently 'wrong' with connecting it wirelessly.

The time difference to crack one over the other is fairly insignificant (like, 8 minutes versus 5 minutes).
Using a 64-bit key would add less overhead (the encrypted key is sent with every packet, which is the main thing that makes it so insecure), so you might consider that if you're forced to use WEP rather than just leaving it open.

One analogy of wireless security is the entryway to a home... leaving it 'open' is like closing the storm door but leaving the inside door completley open; turning off the SSID is like painting the storm door the same color as the house... if you look closely you can still see where the door is. Adding a MAC filter is like adding a lock to the storm door, except anyone with knowledge of those doors is aware nearly all of them use the same key - CH751 or 751CH (which is also the same key 95% of truck caps use), and MAC addresses are easily spoofed. Using WEP is like closing the inside door too, but it's still unlocked (64-bit = wood door; 128=bit = steel door, but they're unlocked either way, so composition doesn't matter much). Using WPA/TKIP is like locking the inside door, but the door is made from glass (with WPA/TKIP the packets can be captured and cracked offline, so your data's still not ''secure,'' but the cracker cannot trace the vectors backwards to decrypt the original password and gain access to the internet), so 'privacy' is not exactly assured. WPA2/AES is a locked steel door with no windows (wireless intrusion detection would be the peephole equivalent).

So in essence, using WEP (and/or MAC filters) keeps out only honest people.