MT3 used underscore to separate names then along comes MT4 and it uses dashes (better choice in my opinion).
Having read some recent articles about non technology specific urls I decided to take the liberty of cleaning them up.
To that end I wrote a script to generate mod_rewrite rules based on the old urls. I also removed the .html extension. So the urls should look something like.
/blog/archives/2007/12/19/movabletype-urls
At least I hope they do.