I've run into 5 total but only 3 new ones seen in March since I installed SP1 on 2/23. None are for SP1 however.
One is security update with another general update for the 64bit Windows, and one other update for the Malicious Software Removal Toolx64. The two seen on the 28th may have been ones missed earlier and unrelated.
Occasionally MS will release a few new ones every so often. Windows Live Essentials, The Security Essentials, Silverlight, Office products, and other things as well as for Windows itself as Shawn was saying depending on what you have on there.
The best place to keep up on security updates in particular is seen at TechNet Blogs
You'll see see a regular update on any new as well as monthly bulletin.