The most recent release was an update/bugfix release only and will be the final 8.04 based TKL. AFAIK the Devs are already working on the new 10.04 based releases. They will probably release initial betas within a few weeks of 10.04 final. They have told me that they are planning to initially release them on SourceForge only and assuming there are no obvious major issues, they will be launched here within weeks of that. At that stage the current appliances will be depreciated (although still available for download via SourceForge for those who want, as 8.04 is supported by Canonical until 2013).

As to your second question, there has not been a definitive answer on upgrade path. As most of the appliances utilise apps from the standard repos, you could assume that many should upgrade easily (as easily as any other server 8.04 to 10.04 upgrade anyway). Some may not so easily where they use upstream tarballs. Read the individual appliance notes to give you an idea of the likelihood of success.

I know the Devs were working on a migration solution but it is unclear (to me at least) how they've gone with that so far.

Despite all that, its probably best practice to backup all your data first (or better still image the whole system) in case things go wrong.