I saw the ST312 in Germany. A stunning display. It was being sold by some German Jepp dealer who jacked up the price by a huge amount, hoping nobody knew about google
IMHO, the GPRS/3G option is worth less for
mobile internet connectivity than might appear, especially for people travelling abroad, because it is useless when airborne, and it needs a SIM card to be internally mounted. The SIM card needs to be administered (contract, or PAYG with some means to monitor the balance and doing topups). I have a Thinkpad x60s laptop with this option, with a PAYG SIM, and it's "nice" but gets rarely used. A heavy user would just get a contract SIM but otherwise it is a hassle, and if you step outside the EU you can blow away the balance rapidly - unless you buy a local SIM card. IMHO, a bluetooth connection to a GPRS/3G capable mobile phone is a lot more usable. The Huawei E585 (and a wifi connection) is another way. You have the same issues with the Ipad, except 99% of those are used within the UK only and they will be on fat-fee contracts; Apple users seem happy to pay almost anything for the constant connectivity.
You could use a
bluetooth keyboard too, though those obviously need batteries. Years ago I had
one for a PDA; rarely used it.
The thing is that once you start talking about keyboards, you may as well get a laptop
There are of course convertible laptops (where the screen folds back to make it a tablet e.g. the Fuji P5010) but again none of those were ever sunlight readable.
Airborne, I've never found any need for a keyboard. There is a somewhat contrived application for one if you have a satphone and then you can do emails...
Like I've said before, it gets easy enough if you divide the job into ground and air. The ground stuff is best done with a light windoze laptop.