Unfortunately I've had to disable the ability to search the comments on this blog as apparently the script was using up too much memory. I ran a search this afternoon and ended up with the generic 500 Server error page. Looking through the error log (which can be found in ~/logs/errors/domain name/http/error.log for dreamhost users) I found that mt search page was being killed before completing but only if I used the SearchElement=both on the absoblogginlutely blog (the other blogs are fine). I must admit I am a bit surprised at that as I only have 3103 posts and 1446 comments so I wouldn't have thought this would put *that* much strain on a server. Dreamhost just said that I was "using too much memory" and to check the MovableType forums for support :-( I did a quick search and that recommended fastsearch, but this doesn't support searching comments either, so for the time being the search will only do the content (although interestingly I can search with no errors from the admin interface).
Hopefully this won't be the same in MovableType4 which I might get around to upgrading too one day.
Update The feature is back - after clearing out the junk comments the feature started working again.
You may also want to try FastSearch for MT, which uses PHP rather than Perl/CGI. It's faster and shouldn't put as much load on your server.
Also: the Google ads in the comments obscure the boxes where I type in my name and email address in IE7.
Thanks for the tip off on the ad's Neil - I had not realised that.
I looked into Fast Search but that does not search comments so it does not help in my case (as it was the comments that I needed to search.