We've seen brand new sub-$100 Android phones from Telstra, Optus and Crazy John's, so it was only a matter of time before that price point also hit older Android models. JB Hi-Fi is selling the Sony Ericsson X10 Mini Pro for $99 outright as a prepaid handset.
Given the choice, I suspect I'd go for the newer models; the Mini Pro keyboard is nothing to write home about, you'll be getting a much older Android version (Danny at Gizmodo has confirmed the device is selling with 2.1), and the phone remains locked to Telstra. That said, it's a lot cheaper than when the phone launched for $629 a year ago. With luck, too, it portends some more Android bargains on older phones in the near future.