finch, The 5790 is now already on the market -- but you must order directly from RIM.
The carriers are not carrying the 5790. Mobitex is more or less a legacy network, and there are patches of customers that need the 5790, but not enough to warrant the carrier carying it. It's a unique situation, because RIM does not sell other models directly -- only the 5790 model.
Some areas, such as Chicago, has an excellent Mobitex network that apparently is better than most phone carriers.
However, in Ontario of Canada, GSM850 now has much better coverage than Mobitex. The new 2-watt PC4 transcievers in the newer 7280 and 7290 often give extra bars of reception, and actually works in rural areas almost all over southern Ontario now (at least outdoors). There was nearly nonstop reception all the way from Niagara Falls to Quebec City on my several train rides earlier this year. That's over 1000 kilometers.
If you stay in North America, get the 7280 or 7780 (the -80 series).
If you go internatoinally, get the 7230 or 7730 (the -30 series).
If you want a world phone too, wait for the 7290 wihch combines the benefits of the -30 and -80 series into one unit;
If you need legacy Mobitex, get the 5790;