Mozilla/5.0 (Windows; U; Windows NT 5.1; fr; rv:1.8.1) VoilaBot BETA 1.2 (email@example.com)
I allow it on the simple string voilabot, which should do for blocking as well.
February there were 23,000 hits across a server of several dozen web sites. That's from a total of 145,000 hits from all permitted bots including google, yahoo, msn (the highest by far), vagabondo, yandex, ask, twiceler and a few others. It's about 12th on one of the more active sites (google is 4th).