Once the new StackExchange sites up, there will be a lot more questions with overlap. One solution would to have the same question asked multiple times on multiple sites, but I think we can have a nicer solution than that. I think that certain users (maybe 5,000k plus) should be able to see a question to be displayed on multiple sites. For simplicity, all reputation gained would be on the origin site. I would expect that it would take quite a bit of backend work to implement this, but I think it would be a really good feature.
Cross posting is a poor choice for a variety of reasons. This sort of thing has been discussed for ages on Usenet, so I won't go into the reasons why it's a poor idea.
However, the solution to the problem you describe, in other words that some questions overlap, is simple:
Choose the site you feel will give you the best answer. Post your question, and let the community give you feedback about the question - they may move it somewhere else if they think it's a better fit elsewhere.
If your question truly does involve multiple sites and no single site can answer it completely, break it up into smaller pieces and post it to the appropriate sites. Just don't post the same question to every site and say, "but please focus on the programming part for SO" - change the question so it clearly focuses on the correct part of the problem for the site it's posted on.
If your question cannot be broken up, then post it on the most suitable site, or choose randomly out of your best guesses. Note that some sites, like superuser, are specifically for problems that overlap multiple areas, so you may find that will fit your needs best.
The percentage of questions that truly, equally belong on two (or more) sites is significantly small. (Granted, those situations will increase a bit as Stack Exchange sites spread.)
The number of questions which don't need any customization to tailor it to the specific community is smaller still.
The probably that users will follow up with every site they cross-posted to respond to inquiries, accept answers, etc. make it that much less appealing.
In short, I think the best policy is to pick the best site to answer your question and start there. If you don't receive a satisfactory response, try different avenues.
I've seen this problem on forum-based support areas that also have technology focuses. All too often users pick the top forum in the list or find the one labeled "chat" and ask their questions in there, only to be forwarded (with varying levels of venom) to the correct forum. An explicit 'triage' queue is an idea I've had, but it still wouldn't solve that kind problem; users will still pick the top/chat forum and hope for the best.
We clearly get users stumbling over ServerFault/StackOverflw and see, "Gee, there are a lot of computery people here. Maybe I can finally figure out how to configure this DSL modem at Mom's place," and ask the question. And get redirected to SuperUser, as that's where such questions belong. The trilogy sites have an advantage over forum-based support options in that users are not presented with an explicit menu of options before getting into a forum, so they just fling their question to the winds of the first site they stumble across. This is an intractable problem, and the current vote-to-close mechanism works well for keeping the venom from splashing on the hopeful questioner.
As we increase the tech specialization of the SE sites we will get more fuzzy boundaries between 'em. SO and SF get a fair amount of quite legitimate overlap on the topic of scripting, and defining the boundaries between SF questions and SE questions is nebulous at best. If the Web Applications SE site gets off the ground, there will be a lot of overlap with SF (PHP/.NET/whatnot configuration/versioning issues) and SE (general application development). It will be up to the various communities to figure out a rough consensus on where to draw the lines. It is perfectly OK to tell a questioner that they'll get more thorough answers on another site, or that one aspect of their question is better answered elsewhere.
This was not considered a good idea, historically, and I haven't seen anything to change my mind on the topic.
- Allow cross-posting of questions to more than one SO site
- Allow cross-posting between serverfault.com and superuser.com?
I am sympathetic to "gee, which question do I ask this site on?" but cross-posting is a destructive and dangerous behavior -- why spam one site with your question when you could spam twenty-three at once?
I haven't currently got time to understand and/or review area51, but perhaps the answer is another new site suggestion: one that is for when you don't know the best site for your question. OT.stackexchange.com perhaps.
All questions are intended to be migrated to the best site to answer the question.