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There are many techniques to sniffing, but it's really about the boundries and access. If your on a company lan, the network adminitrator can sniff what ever port he/she want's to, any time. If it's your co-worker next to you, and they fire up ethereal, cain&abel etc... they might be able to see this info.
Most times brute-force isn't even necessary, there are a lot of protocols that are very very plain-text, and or obusifcated poorly. FTP is plain-text, SMTP is... lot's of stuff that probably should be secured isn't. Sniffing that traffic can be difficult, but there are techniques as I said.
http://en.wikipedia.org/wiki/ARP_spoofing (the links at the bottom, and in the article are good to read also)
http://en.wikipedia.org/wiki/Packet_sniffer
http://en.wikipedia.org/wiki/Man-in-the-middle
-rich