Thanks for the responses--what I've found out so far with some research calls is:
Virgin (only Newark, no Philly) wants you to book your pet first, then make your pet's reservation AT LEAST two weeks in advance--can be months in advance.
British Airways (Philly and Newark) wants the opposite--you book yourself and then NO MORE than 10-14 days before your flight, you call and book your pets in.
Right now BA out of Philly seems to be running @ $200 cheaper for the dogs, as well as me...so it's looking like I might go that way...even though anyone who flies out of PHL knows that you don't always get service with a smile

Gotta love Philly!
I called Continental for flights out of Newark and it runs about $100 cheaper than the other flights--they charge by your pet's weight + crate weight, not volume like BA and Virgin seem to. It just seems like a bit of hassle, as you have to book the animal handlers in the UK that then take them through the procedure on the other side of the pond.
Any more experiences welcome!