I posted about this on the "high protein - more urination" thread but wanted to post it separately, too.  With my 2 puppies after spaying they were fine.  When I had my older whippet spayed (she was 6) when she came home she urinated more frequently for about a week.  I thought she had a UTI...checked with vet....she didn't have an infection.  The irritation of the having all those organs moved around, plus apparently the "odd" sensations she was feeling afterward caused the frequent peeing.

We've experienced this now with our foster, Alfy, too.  She was spayed last Monday and finally today she is peeing with less frequency.  Now, Loco was older and Alfy, although young at 2-3 years old, did have at least 1 litter of puppies....this may be a contributing factor.

Of course, you should always check with your vet when you see frequent urination.  But I thought that is was interesting that there could be another reason.  Found this conversation on the web:

http://www.dogforums.com/13-dog-health-questions/12353-frequent-urination-after-spay.html

"My vet wants to me to monitor her behavior for a few days. He said sometimes this kind of thing happens because dogs feel nervous (perhaps confused about feeling strange post-op), and when they are nervous they urinate frequently. "

Again, always check with your vet to be sure it's not a urinary tract infection!  But apparently, it is common for dogs to pee more frequently for about a week after surgery. 

I've heard a few people talk about dogs "leaking" after a spay too.  I am under the impression it is something that occurs even when they are sleeping etc...  Has anyone heard of this? I am not sure how common it is. 

That's something totally different, called "spay incontinence".  They loose a little control of their bladders.  It happens mostly to older females.  Also, it seems to be more a problem of the dog not being aware she needs to urinate rather than actually urinating more frequently...hence the leakage when they are sleeping, etc.  It can be caused by the lack of estrogen which weakens the bladder spincter.

http://www.veterinarypartner.com/Content.plx?P=A&A=5&S=0&SourceID=28

My sister rescued two lab puppies about 5 years ago, and when they were spayed at 6 months one of them developed the "spay incontinence" and has to be on medication the rest of her life for it. I don't know how common it is, I had never heard of it before it happened to my sister's dog.