A growing list of communications clients offer a P2P-based Mac option. Perhaps the most widely used comes from Apple itself. Mac OS X Tiger is bundled with Apple's iChat AV, a videoconferencing and IM client. iChat IM is compatible with both AOL Instant Messenger and Jabber Instant Messaging. iChat isn't inherently a P2P-based tool, but it can be set up to establish P2P connections with other users who have iChat, using Apple's Bonjour connectivity technology (formerly known as Rendezvous). This works across an established network or over an ad hoc 802.11b network if your laptop is set up properly.
If you're up for a mixed-media session, another option for Mac P2P messaging is Bitwise IM. Like many other IM packages, Bitwise offers not only messaging but Voice over IP (VoIP) calling, file sharing, and whiteboarding options.
You might also try ineen, a combined audio/video conferencing and IM platform. While it was still in beta at the time of this writing, ineen offers several of the richest features available on a VoIP client, including call transfer, call recording, and speakerphone mode. Exploiting the inherent advantages of the P2P network architecture, ineen offers a unique feature that (theoretically) enables you to include an unlimited number of people in your voice or video conference. (Ordinarily, the limit for Ineen conferences is 10 participants.) Distributed conferencing can handle more visitors, because each participant in the P2P mesh is also a host for 10 participants, each of the 10 participants can host 10 people, and so on.