Most frames (my understanding anyway) is that they are built for a ride weight of 220 pounds as is most normal (not racing) wheels. Touring frames are generally built for about 300 and Tandems around 500. But wheels are the biggest concern, and if your a big guy then look into nothing less then 36 spoke rims or 48 if your really large. If you look at a Tandem wheel you'll see that they have at least 42 spokes.

I'm not sure if those weight numbers are accurate, but remember figures similar to those when a friend of mine that weighed 280 (6' 4" all muscle) guy said that some LBS told him; so I'm relating this to you at least 3rd hand; but I have read similar numbers in this and other forums. I do know he broke a Vitus AL frame after only 8 months then bought a Klein AL and broke 3 of those in a 7 year period, then bought a Cannondale AL and broke 2 of those in 3 years. Now he uses nothing but steel and has a 6 year old Gios he rides mostly and a 2 year old Rivendell that he tours on (with an additional 60 pounds of camping stuff) and these frames have not broken...yet!