I left Lotus before the IBM acquisition, but my next employer, Ziff-Davis Interactive, was bought by AT&T (#1) and then resold to Nets, Inc. (#2) during my time there. After I resigned from Nets (delivering my resignation letter to, weirdly, the same CEO from whom I resigned at Lotus) I joined a startup called Instinctive Technology, which was subsequently renamed to eRoom Technology and later acquired by Documentum (#3), which was then itself acquired by EMC (#4). So every company for whom I've worked has been acquired during or shortly after my time there.  

The pattern continues, according to today's announcement that my current employer, ITA Software, will be acquired by Google.  

#5.  

#1-4 were all good for me, personally, but bad for my projects. Hoping this one is half different...  

[16 July addendum: my optimism got a huge boost today from the news that Metaweb is now part of Google, too!]