I am in my second year as an Associate Professor (with tenure) at a top research university (listed among top 20 in US News & World Report in my field). My publication, teaching, and service records are all very good to excellent. I have also received numerous awards such as Sloan Fellowship and NSF's Career Award. In general, I am happy with my current institution, but sometime wonder if I can move to more prestigious university. There are other personal reasons as well that makes me ponder about such possibility. Would it be a good idea to apply other universities for someone at this stage of their career? How do search committees feel about hiring someone who has a tenure?
"Poaching"—"stealing" a tenured faculty away from one department to another by offering a more lucrative package—happens in academia with some frequency.
Obviously, if the position you're interested in applying for is a tenured position, then there might be a chance that the search committee will consider already-tenured candidates for the position. However, it is not a guarantee that this will be the case. (It depends a lot upon the candidate and the relative "fit." Also, a candidate who is fairly close to retirement may not be at the top of their priority list—simply because that means the search may have to be repeated in a few years anyways!)
One other thing to keep in mind—the working relationship you have with your current faculty may be significantly strained if you handle this the wrong way. You may want to see what you can find out on a confidential basis before committing to applying for positions. Once you've sent it out, it's out there—and could cause problems for you if word gets back to your present department.
You may therefore want to have a discussion with your department chair, but I would strongly recommend not using "these schools are more prestigious" as a reason for applying to them. If that's the only reason you're doing it, then you may want to reconsider, as switching schools is a time-consuming and nontrivial process.
It's rare that a search to fill a tenured position proceeds along the usual lines (post an ad, interview candidates, make an offer). Because of the delicacy involved in poaching, it's usually a lot more "under the radar". Which is to say that if someone hasn't approached you yet, they might not be interested in what you have to offer.
Having said that, one strategy (if you're really interested in moving) is to drop hints in the right places with the right people. Contrary to what aeismail says, I'd argue that "moving to a much higher ranked place" is probably the ONLY reason to move that won't create hard feelings (in that people usually can understand the desire to move up the rankings). Even then, the shift should be significant, else moving between universities that are similarly ranked doesn't make a lot of sense unless you have other reasons (geographic, two-body, etc).
I realized I didn't answer the question in the title. The short answer is: it's very hard, but not impossible. It depends much less on the availability of positions in general, and much more on the specific match between the institution and you.