I am by no means a lawyer... but in my opinion there are no _major_ legal problems.

Development of Jabber is completely open. See http://www.jabber.org/ for lots of information. (XMPP is the IETF name for the Jabber protocol.)

AIM/ICQ (oscar), MSN and Yahoo! are all closed protocols. This means the specifications for how the protocol works must be pieced together using packet capture tools such as tcpdump and ethereal.

Decompiling their programs is basically copyright infringement. It's equivalent to looking at their (closed-source) source code and taking ideas from it.