To answer your questions, they are part of the same HX900 series and are essentially the same TV. HX909 just indicates the addition of an RS232 port. HX900 was what Sony was referring to the model as earlier in the year, but now it seems US/Canada will get the HX909 upon release.
Note that the price you saw was also listed on another US e-tailer in the beginning of the year and then removed, I don't think that that price will end up being valid when it's finally released.
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