@anabelle
In the folder for your theme called 'home' there is a file (that I don't have in my default 'home' theme) called last2blogs.php
Inside that file there is code that is looking for a function called list_all_wpmu_blogs. That function could be in the same file or somewhere else (an included file).
So before your upgrade, there were files that you has added (last2blogs.php was one of them) that have been deleted as part of your upgrade. If you have the file backups of your old setup, you could to find where the other files were that last2blogs.php needs.
Maybe there are helpful comments at the top of last2blogs.php which will give you the authors URL so you can get the latest version/reinstall the files etc...
@daveyfire
Same kind of thing for you...
Keep in mind that it is entirely possible for two different people to name their two different projects the exact same thing, so check your file's comments for a starting point...