Are you saying that the gaps between Ubuntu LTS 6.06 and Ubuntu LTS 8.04 are smaller than the gaps between RHEL 4 and RHEL 5? They both have about a 2 year gap between releases.
Well, that's 22 months for Ubuntu LTS. RHEL4->RHEL5 was 25 months. It's looking as though RHEL5->RHEL6 is going to be significantly more than that. (Whatever happened to "predictable release schedule" as a selling point for RHEL?) In my experience, I can trust Ubuntu non-LTS releases more than I can trust a new Fedora "bleeding edge" release. So, if the need arises, I can feel reasonably comfortable upgrading a client from an Ubuntu LTS release to a non-LTS release, with the intent of upgrading to the next LTS release when it becomes available, all without being uncertain about their future upgrade path or security patch coverage.