Same thing happened to me. Everything was working fine though. I did try renaming the folders to their proper names and everything is still working correctly.
Thanks to everyone who replied. We've been further testing the import process and would like to report back on our conclusions regarding these folders in case anyone else finds this thread.
As far as we can determine the folders with the funny names are left in place as a relic of the import process, and can be safely deleted (or perhaps moved to another location just in case).
We've repeated the import process a number of times and each time everything has worked fine with no errors, and the store has functioned as expected following the upgrade.
Each time these folders with funny names have been left in place, but are essentially empty and seem to related to previously used image folders. Moving them out of the site root the store continues to work.
PS) Speaking more generally we've been testing the import with a large, complicated store with lots of products with complex options and many orders, and it seems very solid. Good to see!
Hope this helps some and thanks once again, Happy Fruit Bat