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Your assumption that url: mydomain.com will show the number of pages that you have in Google is not quite right, since Google will exclude multiple occurrences from the same site and include external sites that contain the url.
I hope this answers your question.
How about "keyword site:mysiteurl"? That seems to find those pages on your site that are in Google. The problem is, for it to work you have to have a keyword that is contained in every single one of your pages. Just "site:mysiteurl" does not work, you need the keyword. Still, I don't think it's a problem, slap some generic disclaimer at the bottom every page and there ya go.
But I guess this is not news to most of you.
After it gives you the results, click on the link at the bottom that says "Repeat the search with omitted results included."
That should give you an accurate count of the number of pages indexed.
For AltaVista, use host:yourdomain.com
Exite use site:yourdomain.com
For domains with generic words, a slightly improved workaround is:
allinurl:genericword.+com
This will strip out some of the irrelevant results. It's not perfect, but it's an improvement.