New models can be released throughout the year, but it seems to me that most are released in the summer or fall, and this year's models are already in stores for the most part.
Each display technology- whether DLP, LCoS, LCD RPTV, LCD flat panel, or plasma has its own inherent advantages and disadvantages. One primary advantage of rear projection TV's has been a tremendous price advantage compared to flat panel TV's of similar size and quality. Flat panel displays are "sexier" (and even more practical for people with limited space) so they're very popular- and since they're popular with buyers they're commanding more and more showroom space. As prices for flat panel displays drop one advantage of RPTV decreases so RPTV's drop in price to maintain their relative value.
Some predict that prices will drop to the point that RPTV's will no longer be profitable to manufacture and sell and they'll eventually disappear. Others feel that RPTV technological improvements will continue and they'll keep their value compared to flat panels. I think that you'll find that most posters on this RPTV forum feel that for now at least and for larger screen sizes at least the "bang for the buck" from RPTV's makes them very attractive. I wouldn't hesitate to buy one.