Hey Brian,
A good measure of the "faith" behind a release is whether or not DotNetNuke.com is running it. Right now we're recommending that customers use 4.9.1 because of a security hole it plugs even though it does have a caching issue present.
I'm pretty sure the core team is putting 5.0 through some pretty solid tests, however, a big thing to keep in mind is that 5.0 has been in development for more than a year, and, even though a casual user won't see a lot of improvements, a lot of things have changed under the hood, and, unfortunately, those kind of changes are often the ones that take the longest and are the most under-appreciated.
Once DotNetNuke.com is running 5.x and we've done a number of tests in our environment, we'll start recommending that customers upgrade.
On a side note, PowerDNN.com is running a site that was originally installed on 6/7/2006 and started out on DotNetNuke 3.2.2. I don't see any reason why the corp would decide make 5.0 a platform that requires a rebuild.
Also, don't be too hard on the Corp right now. I know that it sucks getting a problem release, but I can imagine that right now they're going through a large restructuring because of their recent VC funding. Now that financial resources are not an issue, I expect that the community will start to see a major increase in the QA process as well as a more rapid feature release. I expect that 4.9.1 / 5.0.0 will be one of the last releases with anything resembling major bugs.