In the Christmas spirit, the Jabber Software foundation recently released specifications extending the XMPP instant messaging protocol to support voice & video. This provides XMPP developers with an alternative to bundling SIP into their apps for rich media services.
While XMPP hasn't seen much success outside of the U.S. government and financial services firms, the adoption of it by Google for its GoogleTalk service provides an opportunity for XMPP to become a stronger alternative to the SIP/SIMPLE set of standards. Especially with the pending integration of AOL and Google's IM services (see Light Reading: AOL/Google: VOIP Buddies).
Even with Google and AOL coalescing around XMPP for rich-media services I'm still not convinced that there is a real challenge here to SIP. In the consumer VoIP space all offerings that I'm aware of (such as Vonage, CallVantage, Packet8, etc.) are all based on SIP. In the enterprise space all VoIP system vendors and providers of hosted "IP Centrex" services are implementing SIP (though most enterprise systems vendors still rely on their own proprietary signaling standards for end-points). IM vendors such as Microsoft and IBM Lotus are on the SIMPLE bandwagon for integration of presence and IM with voice and video.
Still, the emergence of XMPP alternative to SIP/SIMPLE will be interesting to watch, especially with the power of Google behind it.