Can You Integrate User Databases Across Multiple Domains?

While I know there are ways to integrate two user databases together (say pligg and vbulletin), but is it possible to accomplish this across domains?

Here's the idea. Lets say I was going to create three sites that all had a common theme that made going from one site to another almost unnoticeable. This is done for branding purposes.

1) FishingTips.com - wordpress blog where fishing tips are published

2) FishingSpots.com - pligg style voting site where users can post their favorite fishing spots and others can "digg" them up or down after visiting.

3) FishingTalk.com - a vbulletin forum for fishing enthusiasts to talk about their favorite sport.

All three sites are "mashed" together by tabs at the top of the page that say:
Fishing Tips | Fishing Spots | Fishing Community

Now it would be nice to integrate the forum and voting site's user databases together so once someone registers for the community, they don't have to re-register to participate in voting on fishing spots.

If they were hosted on the same site it wouldn't be an issue, but as the sites are on different domains is it still possible to do?

 

There are a few ways to do this, none of them that good.

1) Store a client side cookie for this. You REALLY don't want to do this as it's easily hackable.
2) Store session information in a database. Much more complicated as you have to manage all the session state stuff with your DB, but this probably works the best, but will increase performance load as you have to hit the DB more now.