As has been widely reported, Microsoft has released two important patches for Vista that improve the compatibility, performance and reliability of Windows Vista. Given the scope of these updates, you could argue this is a Mini-SP1, although that's not how Microsoft is positioning the updates. I've downloaded these patches and my Vista system seems to be working OK.
The first update is described in KB938194 and is intended to improve compatibility and contains fixes for 13 identified issues. This patch can be downloaded for both i386 and x64 platforms. The download is just over 2mb (or 3mb for X64).
The second update is described in KB 938979 and is intended to improve performance an reliability. This patch contains fixes for 10 identified issues. Like the first patch, you can get i386 and X64 patches from Microsoft.
These updates look useful in improving Vista's performance, compatibility and reliability.