hmm.. i'm not crazy about url rewriting, i think that's a rathole we don't want to get into.
What worries you? SMF doesn't seem to use /showthread.php or /showwiki.php, which would of course be a problem if they did. (edit: actually, I'm not sure even that would cause a problem, since there's also topic vs thread and msg vs post to disinguish them).
that said, i noticed that the previous urls had topic names in them, which is an seo technique for increasing relevancy, and could speed up re-indexing (because frankly that's how most people find content these days). SMF has a similar toggle that can gussy up URLs; i'll try flipping it on now and seeing if that helps things a bit.
Would URLs without topic names in still work as well?
edit: technical complications prevent this. will have to give this issue more thought.
Prevent which? Adding topic names, or redirecting URLs?
Redirecting URLs wouldn't just connect us back to the web, it would also fix links between threads. And I think there are a remarkable amount of those!
I think I'll try writing a greasemonkey script to patch the URLs. Obviously it would be 100x better to do this on the server! But greasemonkey will be a good place to test out the regular expressions required.