The optimist in me hopes that Amazon knows something about a successor and/or sequel to the G1. We know that the HTC Magic, also an Android phone, will be released on Vodafone's network later this year. Is it possible that Amazon is clearing stock because the Magic is set to go on sale in the States at some point in the near future? Could be.
Right now, only the black model is receiving the fire-sale treatment at Amazon, however. The bronze and white G1s still cost a buck eighty. Is the black version of the G1 not selling well compared with the white and bronze models? Also a possibility.
There are rumors suggesting that the G1's successor will launch at the CTIA Wireless event in a few weeks. This hasn't been confirmed or hinted at by HTC, of course, but it is within the realm of possibility.
In the end, if you're looking for a deal on a black G1, you can get one from Amazon.com at the moment. A two-year contract is still required. Happy shopping.