I have some first hand experience (and plenty of 2nd hand knowledge) with some animal charities. I can agree that the bigger ones do tend to spend possibly too much on awareness campaigns, 'stuff' and donations could possibly be better spent on smaller, more local organisations - but if the bigger ones accept BTC then the smaller ones are more likely to pick it up too, not the other way round.
So I think its just as important to push the big ones to get on board as it is to give donations to the smaller ones to get more effective aid for the money (however every charity has admin costs and rely on marketing campaigns to generate more donations - the smaller ones do not get gvmnt / public money, at least none that ive ever been involved with - obviously its different everywhere).
In any case, if the ASPCA start to take BTC donations im sure it wouldnt be money badly spent, even if it goes to a commercial or some other PA stuff - awareness is important too, of both animal charity in general and the money that makes it to the 'front line' (for every ASPCA commercial out there, im sure other charities get a boost from it too).
I know a few local charities do a great job worth a mention (that I also contacted about BTC before), just in case people want some suggestions for BTC mailing...
Dogs Trust
http://en.wikipedia.org/wiki/Dogs_TrustCats aid, Ispca and
http://www.bluecross.org.uk/