Thanks for the suggestion. Our server is already cached to the hilt, especially for guests (we use Varnish, though, not memcache.)
Ahh, good choice in Varnish.
Course, the problem with a big site is that while, yes there are lots of guests, there are also more members too - and it's hard to cache dynamic pages. It's possible though, but not without hacking vBulletin to pieces like I have done.
Keep up the good work and if you need any advice or suggestions on hosting, server setup, caching and all that jazz, drop me a line.
(I'm a system engineer by trade who also happens to run a very popular site on the side)