Welllll, we've put a lot of work into this version, and it's pretty different from the Win/Mac/Linux versions, so... yes, we're asking people to buy the Android version separately. This is also one of the reasons we launched with a sale: so fans who were waiting could pick it up at a reduced price.
Finally, there are new people involved (ie. getting a % of revenue) in this version, so we want to be fair to them as well.
If your transaction is failing due to a Market error, you might want to try purchasing the game again after waiting a few hours (and/)or contacting Market support. All we can do is check the status of a transaction or cancel it. Their email address is android-market-support [at] google [dot] com, or you can get live help with the "talk to a specialist" feature which is fast and open 24/7: http://support.google.com/androidmarket/bin/request.py?contact_type=contact_policy&policy=apps.
Occasionally people will get this error and find themselves unable to download the game temporarily, but it clears up with a little time or Google support magic.
I'll admit, at first I didn't think it was fair, but your argument makes sense, new development costs can't just be absorbed. FYI I'm going to buy it for my phone right now, keep up the good work guys.