Please note, this is in no way limited to Sony Ericsson's foray into Android - they did the same thing with their UIQ handsets, as well as their Symbian-powered devices. They've always been horrid with updates - quite honestly, the other manufacturers - Samsung, LG, Motorola, etc - are the same as well.
I always found it interesting that people bashed Nokia's process for updating phones (until the past 6 months or so, it required a complete wipe of your phone if you installed a new firmware). However, Nokia, along with perhaps HTC and Apple, are really the only companies who are actively supporting their older handsets. Granted, not all devices will get the love, but on average, most do. Hell, the N97 Mini just got a firmware update last week, I believe.
Samsung has been promising their Galaxy S users an update to Froyo since the freakin phone came out, yet I'm nearly certain that there is not an official update out there.