Hardware obsolescence will eventually halt fragmentation on its own. Most consumers roll over their device at the end of a contract anyway, that's just 2 years or 3 in some markets. Anyone that holds on to a phone for over three years either doesn't care about bleeding edge updates, they're happy with an old device anyway, or they've extended their device's life via custom roms. In other words, If it is January 2013 and you're still on Eclair then a) you probably don't know what 'Eclair's is and b) you're not losing any sleep over being excluded from updates.

I don't see a huge problem with the android market being 'fragmented' over 2-3 versions. It's all just optics: Android haters trumpet the fragmented platform while advocates brush it off. It shouldn't halt or slow down progress, and if it does then Google risks stagnation.