I think you can register up to three CC's per iDine account. Each airline program is considered a totally separate iDine account. Therefore, 5 airlines with 1 CC for each is no problem.

In fact, if you visit two or three iDine restaurants VERY frequently (i.e., lunch spots, neighborhood taverns, etc.), it's probably worth it to pick up as many simple no-fee credit cards as you need to max out the miles in the various programs iDine is hooked in to. I use 3 cards attached to UA, AA, and DL to earn quite a few "free" miles each month. I'm just not quite a big enough spender to switch to the 20%-discount program.