Apparently the Windows 7 SP1 is less than 150MB in size, which is good considering that service packs for Windows Vista have been much larger than this.
The build numbers for the SP1 is rumored to be somewhere between 7600.20500 and .20700. A beta release is expected in July 2010, so you can except a lot of changes to happen in between now and then.
Apparently this build fixes over 150 issues. However, users should not be troubled about the late release of the SP1, since Microsoft usually releases regular patches through Windows update. The SP1 contains most of the released patches and new ones in a single bundle.