If you were among those who installed CU4 for SQL Server 2008, then installed SP1, you may not know that you lost some of the fixes that came with CU4 (see Bob Ward's post which is slightly more informative than the release notes).
Microsoft today released a post-SP1 cumulative update that adds back all of these fixes. There is nothing else included, so there is no reason to install this CU if you are at SP1 and got there from RTM or one of the CUs. There is always some confusion about the servicing model and what to do when the cumulative update and service pack schedules collide. Here is a summary in this case (the most important one being the first item):
How do you tell what build you upgraded from? Well, hopefully you remember; if not, you should be able to tell from the most recent set of setup log files in "%Program Files%\Microsoft SQL Server\100\Setup Bootstrap\Log\" … the file summary.txt has a section called "Product features discovered" and you should be able to find the instance(s) you patched last time, and the version will be listed under the section "Database Engine Services."
You can download the new CU here:
And from there you will find links and descriptions of the list of fixes.
If you're still confused about the SQL Server servicing model, don't worry, you're not alone. Browse Bob Ward's blog starting with the link provided above, and also see KB #935897.