I've just been playing around with 2.7-beta3 and attempted to import some posts from another installation of 2.6.3 but there's some issues when it gets to importing the tags for posts.
All of the tags import correctly into WP but they are not assigned to any posts. When you browse the tag listing they're all there correctly but with a tag count of zero, going into a post shows they are not associated correctly as well.
Here's where it gets strange, in addition to importing all the correct tags it creates a bunch of numeric tags (1, 2, 3, 4 ... 341, 342, etc.) that is equal as far as I can tell to the number of legit tags that are imported. It is these numeric tags which are associated with the imported posts.
Categories appear to import correctly and posts are assigned to the correct categories.
This can be duplicated by taking an WXR export and importing it into 2.7-beta3 or alternative just throw up a 2.7-beta3 installation, create a post or two, then export from 2.7-beta3, create a clean install of 2.7-beta3 and import the WXR file you just imported. It will import with the strange tag issues.