What does is have in it? Quoting from the from FAQ:
ColdFusion 8 Update 1 provides full 64-bit support for several new platforms including versions of Windows, Mac OS X, and Linux, in addition to Solaris (which was added with the initial ColdFusion 8 release). We have also improved functionality in several areas including AJAX functions, CFPDF, and CFIMAGE. We have also updated several software libraries.
Full details on these and other enhancements/fixes are available in the Update Release Notes.
Short answer: Lots of goodness.
To answer one question I have already gotten, there is NO update to the LiveCycle Data Services 2.5.1 bundled in with ColdFusion 8. Nor does the update contain BlazeDS. There are instructions that come with BlazeDS on how to integrate it with ColdFusion (see the resources/ColdFusion directory in the BlazeDS distribution).