The MSExchange team blog has announced what effectively amounts to a recall of the initial Update Rollup 3 for Exchange 2007 SP3. Apparently there was a risk of database corruption with the initial version which they say has now been fixed with the newly updated version of the same Update Rollup.
If you were one of the first adopters of Update Rollup 3, you should read what they have to say on how to address the issue.
You can find the details at their blog:
Saturday, April 02, 2011 5:35:00 PM