deadsea - 11:16 am on May 23, 2011 (gmt 0)
We often get reports here from a webmaster that has all their sites kicked out of the Google index at one time. Google certainly looks at many sites in the same bucket.
I'd imagine its not just hosting IP address that Google looks at though. I'd imagine also:
* Site template
* Google webmaster tools ownership/verification
* Google analytics account
* Google Adsense account
* Site link profile