The services themselves don't seem to be changing at all, it's really more of a destination change to make it easier for users to find the Google media stores rather than having to go through other products. This means that your old bookmarks for the Android Market Web store will automatically be routed to the new Google Play address, although the Market itself still looks the same with sections for Apps, Music, Books, and Video. Similarly, the Google Music website has been pulled under the Play subdomain. The changes should be rolling out to Android 2.2+ devices "over the coming days".
As part of the re-branding, Google is also offering a new album, book, video, and Android app at a special price for each of the next 7 days. Today's deals include Android game Where's My Water, the ebook of Extremely Loud and Incredibly Close, a rental of the movie Puncture, and the That's What I Call Music #41 full album, each is only 25 cents.