weird 404 problem

hello all, three weeks ago we’ve upgraded from WPMU 2.9.2 to WP 3 multisite.
since then we get NOT FOUND crawl errors in google webmaster tools, google claims we have urls which doesn’t really exist, for example:
a blog which only has 3 posts gets thousand of Google Webmaster Tools error
do you have any idea, what could go wrong?
here’s an example:

URL Detail Linked From Detected
http://saloona.co.il/healing/?p=17949
404 (Not found)
unavailable
Feb 12, 2011
http://saloona.co.il/healing/?p=17961
404 (Not found)
unavailable

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