Just to note that my own Calendar code Calendar Lite and Pro are having the code ported across so they are compatible with BOTH phpBB2 and phpBB3. Hard to believe I started writing the code 6 years ago!
In terms of development I'm in the process of writing an abstraction layer that allows the use of the bulk of my code logic to be used in common across both versions of phpBB2 but it's a slow process that involves a lot of work.
Once that lot's done I then need to create templates specifically for phpBB3, update the language files to be compatible and then the dreaded ACP changes... before finally writing an installer that can handle not only the Calendar data upgrade to the latest version but also for anyone upgrading from phpBB2 to 3.x
In terms of functionality there's a lot planned to go into the Pro version while the Lite will be getting a much needed code update albeit feature lite (as per the name).
If you want to keep up to date on changes or code then drop in on http://www.snailsource.com/forum/
and check out the Calendar Dev and Snailsource2.0 forum spaces for more info...