Question: Specifically stackoverflow and serverfault.

Is that possible? It sure would be convenient for scripting purposes.

Answer: 
We had a vision of a unique OpenID identifier as the "key" to all three sites, but Google's "every domain gets their own hashed OpenID" has kind of thrown a huge monkey wrench into that. :(

Edit: we now have fairly robust cross-site account association that even works with Google (see your accounts tab), but I realized I didn't answer your question. We never planned to provide for the same *numeric* ids for every user.
I would LOVE to see this happen. I realize that it's probably too late in the game to change the user IDs on ServerFault, but back when it was first announced I had the idea that a user would have a single account (with, incidentally, a single ID) that existed on all the sites. Separate posts, rep, badges, etc. but basic user info would be the same across the sites.

Not only would it have made it easier to provide a unified rep tracker, but it would also eliminate some of the headache that goes with migrating questions from one site to another and trying to preserve ownership of the content.

However, it looks as though Jeff is trying to right this mistake with Meta by prepopulating the site with the current users from SO and SF, and is [actively looking into](https://meta.stackexchange.com/questions/2/sync-meta-user-accounts-daily-with-other-family-sites) keeping the data in sync. So I guess we can probably expect that SuperUser and whatever other Overflows come down the line will enable users to retain their IDs from whatever sister site they're a member of.