There is quite some overlap between SE sites now, e.g. between AskUbuntu and Unix and Linux, or between Ask Different and Superuser. As more sites launch there is only going to be more overlap.
This is not a bad thing, but it can lead to experts missing questions they would be able to answer, just because they are asked on a site they don't watch regularly. The solution to this are the tag filters one can define on stackexchange.com.
This is a very useful feature for SE power users, but I suspect that most casual users don't use tag filters to follow questions across all SE sites.
One way to raise awareness of questions on other SE sites that could be on-topic on other sites would be to advertise them in the sidebar. For example, questions tagged ubuntu on other sites would be occasionally shown in a sidebar on AskUbuntu like this:
Ubuntu questions on other sites:
- How to install game X under Wine on Ubuntu? (Gaming.SE)
- Is Mark Shuttleworth plotting world domination? (Skeptics.SE)
- Make the terminal colorful and blinking on Ubuntu (U&L)
It could look like the box in the sidebar advertising the featured questions on the site meta. The title would lead to the tag filter on stackexchange.com, where the user could add this filter to his account.
This could also be a full tab under "Questions" on the site besides "newest", "voted", ... but this feature would probably not be important enough to use this prominent space.