I shipped them via Continental/United and it was a huge headache.
Because of Continental and United merging, there is a bunch of drama.
I did everything I was supposed to do, taking my cats to the vet, filling out all of the paperwork and drove to my state capital to get the forms stamped by the state vet all within 24 hours of flight. (Must be 24 hours when pets are flying unaccompanied, which mine did.)
I show up at the airport and it turned out my reservation was lost.
I had the confirmation email, but it was gone.
I put my cats through that hell and drove all over the state (13 hours total) for NOTHING.
6 days later I did it all over again for the new booking and it was successful.
My cats are safe and sound at my new home in England and I will join them in a few weeks!
I would like to caution anyone sending pets in the near future.
I found out the hard way that the EC998 is no longer used.
They now use the Annex II form and the APHIS 7001 form.
My state (Michigan) USDA office was very helpful. They sent me the forms via email, including a cheat sheet with how to fill out the very complex Annex form.
I did my part and then took it all to my vet because these forms are new to them as well. (As of this year, March 2012 the EC998 is no longer accepted.)
I was also in touch with an agent on the UK end who sent me forms via email that he wanted filled out and scanned and sent back, which I did. He was very patient with my millions of questions.
I can send any of these forms to anyone who is interested. Just message me.