You can take the boy out of file-sharing... As already covered by a number of bloggers who stayed up later than I did last night, the new version of KaZaA (v3.0) comes with Skype bundled in, as well as a free trial of TypePad. KaZaA claims 300m downloads to date, and the data I have seen suggests something like 4m concurrent users on average, so on the face of it this has the potential to expand Skype's footprint considerably.
Then again, concurrent P2P user numbers on all monitored platforms have more than doubled in the past couple of years, and all of this growth appears to have come on platforms other than KaZaA. As part of a project, I have been painfully aggregating download data on the top 10% most active BitTorrent applications on SourceForge.net, and the number I come up with is 67m since it was created in late 2003. The Azureas client alone has seen 27m downloads in 16 months. KaZaA has lots of users, but the growth segment of P2P is BitTorrent and eDonkey variants (eMule is at 77m and doing 4 - 5m per month), as far as I can see.
I would also expect that KaZaA users generally have a pretty high awareness level of Skype as it is, so I don't know how much of an incremental uplift we can realistically expect to see. Conversely, maybe Skype's view is that there's no point in preaching to the converted (I assume all BitTorrent users are aware of Skype), and this is a clever attempt to penentrate a less tech-savvy audience, i.e., recent converts to KaZaA.