I am using an IBM ThinkPad running XP Pro.
I used to have the same issue on a Toshiba laptop years ago.
Everything used to work just fine, but recently (I think after installing a PCMCIA wireless internet card from Verizon, but I'm not sure) it started randomly taking a lot more time to resume out of standby mode, and sometimes it works right away.
To resume, I hit the FN key. I notice a sound right away that tells me that the laptop received my "request". It sounds like the cd drive receiving power (not spinning up, but a little twang sound). Then the power light is supposed to come on which sometimes doesn't happen for 1-3 minutes, then the hard drive should be doing stuff, somewhere in between the pcmcia card is receiving power and starting to blink, then the monitor comes up.
That used to happen within 5-10 seconds, now it can take 5 minutes.
Since it started happening I have updated all my drivers without success. Windows is always updated. The hard drive seems to be OK, no errors in the Event Log, and the only thing that now is starting to act up (months after the standby issue started, so it may not be related) is that sometimes my networking control panels freeze or the built-in wireless-b card does not see any access points. These are issues I can deal with separately and not part of this question, but maybe someone thinks they are related and helpful to know of.
Any ideas where I can start to search for the cause of this?