Are you saying you have a cluster of servers sitting on multiple OC-255s?  Like I said, if they can actually fill the pipe with SYNs you're pretty much screwed, you need help from your backbone provider.

That said, I haven't seen any significant detail divulged on the attack.  No one's said what they're attacking, the IP stack, the application, what.  Are they overrunning the pipe or server resources?  If the latter there's more than likely a few things you can do to scale just to make it harder for them to effectively DDoS you.