Yahoo! has followed Microsoft's lead and blocked access to its instant messaging service from third-party clients.

Yahoo! set a deadline earlier this month that the protocol would change. Like Microsoft it's happy to license the protocol for use in third-party software, although Redmond's plans are both more formal and advanced (its announced a certification program for third-party MSN IM clients).

The move affects many users of software such as Cerulean Trillian (Windows), Fire (Macintosh) or IM+ (Symbian phones, PocketPC) who want to consolidate the major instant messaging services - AOL, ICQ, Yahoo! and MSN - into one client program.

Cerulean, makers of Trillian, last week promised user a patch. Macintosh users will need to download the latest version of patch, released this morning, from SourceForge.

Whether we'll see a repeat of the cat and mouse games of four years ago remains to be seen. AOL made frequent modifications to its OSCAR protocol in a bid to prevent third-party AIM clients from using the service. It eventually abandoned the tactic, but not through lack of trying. A bootnote to the FTC's approval of the AOL/Time Warner merger requested that AOL ensure its IM services more accessible to third parties.

This time, there's no regulator in shining armour to remind Yahoo! or Microsoft of any such obligations. ®