Well, if you have 2 'halves' of the party that are at really different optimization levels, then yes, it could be a thorny issue. I've had one or two optimized characters in a party of 5 or 6 PCs and it didn't seem to cause big problems. I'd say the worst issue was the optimizers were a bit impatient with the other players not PLAYING as tactically as they did. Everyone still got to do their stuff though and they didn't dominate encounters, just did a bit more effective attacks and etc.
One thing that I'd think about is how much other non-combat aspects of the characters overall matter. Not all optimized characters are hapless out of combat by any means, but if they are REALLY optimized they won't have spent any resources on anything much else. The more generalist characters could easily be a bit more capable in situations outside of a melee.
This is one reason I regularly keep putting the party in situations where using skills and utilities and misc sorts of items can really make a big difference and the stakes are pretty high. It tends to discourage the 'all-in-for-combat' sort of character builds. That way you will STILL have people that are better builders and have an edge in combat, but they will probably have to use a couple feat slots and maybe a power or two for other stuff, which can at least hold them back enough to keep things more balanced.
Of course you could also give the weaker players some extra help with a bit nicer item or whatnot. I don't know how that will play with your group, but if it brings people a bit closer to parity it should work. The optimizers are still BETTER, but not so much that they do 2/3 of all the damage.
