I don't believe this is true. Service dogs these days can be any breed. I don't see any common service dogs on the AKC's recommendation page for breeds which are good for allergy sufferers:
http://www.akc.org/about/faq_allergies.cfm
Ugh, good thing I haven't had to deal with them yet then. Hopefully my track record will continue. My dog allergies tend to be just nuisance allergies anyway. I feel bad and get the the typical reactions, but it isn't like it is with cats. One pill is generally enough for a dog (unless there are tons of them around), and I can drink enough caffeine to counter one pill.