Pidgin is apparently not so very clever, you have to enter the exchange number with it as well (unless you are accepting an invitation, which Kopete doesn't support) but it does have a more sensible default, which is exchange 4 (rather than 2).
http://www.bigblueball.com/forums/aim-su…-aol-chats.html provides a useful list of exchanges:
Here are some exchange numbers and their properties as known so far:
1 Unknown
2 AOL Private Chat/AIM Encrypted Chat (AIM users can only enter if they have a security certificate for IM and chat encryption)
3 Unknown
4 AIM/AOL (This is the exchange under the English Buddy Invite option found as a result of right clicking and choosing buddy invite on the buddylist) [23 people max]
5 AIM only (This is the exchange used for the hyperlinks on the AIM chatlink pages under the chat and send window. When a room such as "Computing Chat" maxes out it creates a new chat in numbered succession to put you in. Example "Computing Chat 1" "Computing Chat 2" and so forth. [23 people max]
6 AIM=Unknown properties
7 Unknown
8 AIM/AOL=Unknown properties
9 AIM/AOL=Unknown properties
10 AIM/AOL=Unknown properties
11 AIM/AOL=Unknown properties
12 AIM/AOL=Unknown properties
13 AIM/AOL=Unknown properties
14 AIM/AOL=Unknown properties
15 AIM/AOL=Unknown properties
16 AIM/AOL=Unknown properties [48 people max]
17-28 AOL=Unknown properties