We have had many threads discussing this (see [webmasterworld.com...] for example) - and the consensus is always that validation is not part of Google's algorithm. Matt Cutts has also confirmed that on several occasions. However, there are errors that do not trip up a browser's error recovery routines that will still give Google troubles when they index a page.
I almost always run pages through a validator. When updating I use the validator in Homesite -- and when first creating a template, I use the W3C Validator [validator.w3.org]. I have been burned by errors that caused parts of pages to be skipped in Google's indexing, even though browsers displayed those sections.
[edited by: tedster at 3:52 am (utc) on July 3, 2007]