Warning :the method presented in this article are too old to be applied now ( maybe work on Mate ? ) . Use this information at your own risk.
I hacked this morning the thumbnailer distribued with the openraster librarie to work with the Krita native *.kra files. I tested this on Linux Mint 9 and 10 ( so I presume it will works on Ubuntu Lucid and Maverick ). Btw, I don't know how the new Ubuntu 11.04 would handle it ( please use comments if you tested it with a success ). I redistribute it under the GNU GPL license V3 ( file inside ), same as the original libora thumbnailer.
Grab this kra-thumbnailer-gnome. zip files (13Kb) , decompress it on your system , run a terminal and move to the uncompressed folder. The code lines to copy/paste are in the 'install' file. that's it ! Enjoy new *.kra thumbnails.
Note: thanks the Krita dev CyrilleB for the thumbnails improvement he made in few minute on IRC(freenode) #krita channel. Before his commit, the thumbnails were dithered. So, if you have *.kra file dithered or not good looking thumbnail, don't report it : it's fixed. If you compile it from GIT master, just update and compile , open your old *.kra file and save them over : you should have the pretty thumbnail now.