Not to mention bad timing,I agree going multi-platform is a risky move for Insomniac.
IIRC based on data in 2009 the ps3 sold ~15% or something more software than the xbox360 worldwide, and in the first three month of this year its sold ~40% more than the xbox360. From 2006-2008 it was the opposite, i.e. the xbox360 was selling a lot more software than the ps3.
Then again perhaps natal will boost the xbox360 software sales but I feel most of those sales are gonna be in the casual game genre which I believe Insomniac arent exactly known for.