Since Calibre is running on the three major PC platforms my guess, and it's only a guess, is that a port to Android might not be too difficult. A bigger question is probably whether it's worth doing. The vast majority of the people who have a use for a program like Calibre have a PC of some sort and the only large scale use of Android these days is on phones, where Calibre probably wouldn't be as useful.
Of course now that I've said that the Android version will likely be released in a few days.
Barry