I've been trying to ignore this rumour on the grounds that it's too unlikely, but now who knows? On March 26th a Morissette fan website reported that:

"According to French-entertainment website actustar.com, Alanis Morissette will be doing her third soundtrack single this year, with Axl Rose from Guns 'N' Roses! The two will record a track together for a NEW Trilogy coming out, called "Lord of the Rings"

Now our own staffmember Susie says she saw an interview with Alanis today on MTV where she apparently confirmed this rumour and added that they would be doing a cover from Led Zeppelin's third album.

UPDATE
Susie found out more. The Zep song in question is "WHAT IS AND WHAT SHOULD NEVER BE"