Yesterday (2010-03-26), the 7z dump for English Wikipedia was completed.
I am downloading it at /mnt/user-store/dump directory, it will be finished
in a few hours (about 4), it is about 30 GB. So, if you need it, you know
where is, don't download it again! ; ).
A tip: in my python scripts, I decompress it on the fly, like this:
7za e -so ourdump.xml.7z | python ourscript.py
And, inside the script, I capture the data with: source=sys.stdin