This HD2b will be t-mobile only, ive come to understand this wasn't even updated by htc, but by (some tech company hired by) t-mobile, it WILL NOT EVER be available to non-usa/non-T-mobile customers.
On the bright side, its been confirmed that the HD2(a) does have some extra RAM, which can be enabled by flashing a custom rom, please understand that flashing isnt for the average joe though, if u are a average joe, please read the tutorials on xda-devs before flashing, if you make mistakes you can brick your HD2 beyond repair (given, youll have to try hard to break it that baldly, but ye, investigate, read tuts, weigh your options, then decide if you really wanna flash for ~100mb of RAM wich wont make a difference for 99.9% of the things u do)
This extra RAM is sayd to be there for when windows mobile 7 comes around the corner and wants a little more memory to meet its recommended specs
Edit: since were on the subject of 'hidden', you should also be aware that ALL HD2's (so a&b) only use ~750mhz of the cpu, this is due to battery reserving afaik, this can also be 'fixed' to be locked at 1ghz with a custom rom, at the price of a little shorter battery-life (like during a day, im not sure what it does to the actual battery life)
Im a FpseCE supporter!