Windows Mobile 6.5 is a negligible architectural change from Windows Mobile 6; it uses the same version of Windows CE (the underlying operating system), all they've really made touch-friendly is the shell for browsing / launching applications. (also improved Pocket IE a bit, supposedly, though lots of people have already switched to Opera) So other than possibly making it difficult to launch Instant Access from something other than a hardware button press, it shouldn't have much of an impact on Pleco.
Windows Mobile 7 is still largely rumor at this point, but supposedly that too will retain largely the same internal design. Windows Mobile 8 is when they're really changing things, I think - Steve Ballmer recently suggested that Windows Mobile may eventually converge with desktop Windows (the catalyst for this will be a new mobile-phone-friendly version of the Atom processor that Intel's cooking up), which would be fine with us since it should be easy to port the Windows Mobile version of Pleco to that.
But it's impossible to promise anything about future versions of any operating system at this point - it's not inconceivable that Microsoft might eventually release a .NET-only version of Windows Mobile, which would require something close to a ground-up rewrite of Pleco, and if at that point 95% of our sales are on iPhone we might not necessarily be able to justify doing that rewrite. The Windows Mobile version could get cast into the dustbin in a few years just like the Palm OS one. The mobile phone industry moves way too quickly to purchase any device as a long-term investment - if you want to always be running the latest version of Pleco you may need to be prepared to buy a new handset every 2 years or so.