I don\'t believe 56k users will make BB users lag. The only problem will be for 56k users playing against BB users. Take SOCOM for example, they only allow BB users to play because ping is everything in a FPS and the average 56k user would just get frustrated, complain, or return the game saying it doesn\'t work.
Frankly it was a smart idea to make SOCOM BB only. When I play Team Fortress you can easily spot the 56kers who come on the server - 200+ ping compared to my 35 ping. I don\'t lag, they do. It also gives BB an unfair advantage if say two people go head to head with the same weapon - the BB guy will always win. Games like this are all about latency.
As far as other games go like Madden - the 56kers will "lag" a little, but the BB people shouldn\'t at all. I have been playing online games for over 5 years and its always the same whether its strategy, fps, sports or rpg. 56kers do not lag out a server - they just lag themselves.