Here's a great blog post from the Office Sustained Engineering feed laying out specifically what is in SP2 vs the April Cumulative Update. It also lays out various installation scenarios and answers lots of questions with the specific Knowledge Base links.
"Many of our customers have questions on the relationship between the set of hotfixes released in the Cumulative Updates (CU) for April 2009 and Service Pack 2 (SP2) for the 2007 Microsoft Office System and Microsoft Office servers. This post will aim at answering those questions."
So, if you are one of those people click thru and check it out. Can't ever have too much information about a major SP release.