Ok so this is not a true faq yet but based on previous SPs I predict it will be soon...
The KB article that wraps the hotfix is here, now a little explanation.
To ship an SP, which is a large undertaking for a product as large as SQL Server, at some point we have to stop taking fixes into it. There are cut off dates throughout the process of release and the calibre of accepted bugs goes up at each stage. However during the development of the SP, we will continue to release hotfixes on top of the RTM code base, some of these may already be in the SP, some may not. At regular intervals during the beta of the SP, the code from the hotfix source code tree is RI'd (reverse integrated, an automated mechanism where code from one branch is automatically merged into another) into the SP tree, however at some point even this has to stop to allow the SP to ship.
Hence some weeks after an SP we release a rollup hotfix which includes all the hotfixes which have been released since the last RI and that also apply to the SP. This also gives an opportunity to address any key regressions that exist.
As part of the more open, frequent release process we have been following, the rollup hotfix is publicly available this time (that is you do not have to call CSS) and hence went through multiple rounds of additional testing over and above what a normal hotfix would get.