Yeah, development is really taking off already, thanks to the huge amount of development out there for other Galaxy S devices. The Charge is looking to be pretty popular for awhile, and all it takes is a few devs who know what they're doing to give you some great options. Read
this if you want to know how easy it is already.
As far as keeping your unlimited data, I have no idea. I'm glad that they're allowing customers who already have a smartphone with unlimited data to keep that data package when they upgrade, but I have no idea about 3g to 4g. So far, they haven't made any distinction whatsoever between 3g and 4g data, and the fact is, LTE is cheaper to operate and has better range and signal penetration once the network is up, so it really wouldn't make sense for them to act like its a premium option at any point, its just a great feature. So in that sense you may very well be able to upgrade from unlimited 3g to 4g without changing anything.
But I just don't know, considering the much greater bandwidth yet much smaller footprint, they may start to distinguish the two. When the T-Bolt first launched, they acted like offering unlimited 4g for the same price as 3g was almost a promotional thing, though they've done away with that talk now. Yet considering its greater bandwidth, they may start to run out of backhaul, in which case they'd definitely start trying to keep the number of 4g users down. Or they may offer higher caps for slightly higher price points, considering the fact that 2gigs is nothing for a 4g device. Or once they have a strong enough LTE footprint, they will want as many people as possible to switch, and may start offering higher caps for the same price. Any of those scenarios would differentiate the plans, and likely require a plan change.
So at this point, no one has any clue. It sounds like you'll be able to do that, but you definitely want to follow them closely in case you hear anything that may suggest they're distinguishing between 3g and 4g data. And they may do that from day 1, in which case you'd be screwed. I went with a Charge because I didn't have a smartphone at all, and saw no reason to get a 3g smartphone at this point. I thought about getting a DInc off ebay and using that for awhile, but I didn't wanna risk not being able to upgrade from 3g to 4g, and I also don't know if you'd be able to upgrade like that if you never bought a smartphone or signed a contract with smartphone data with them in the first place. They wouldn't have much incentive for someone who bought unlocked and was never under contract with their smartphone.
Regardless, all we know for sure is that anyone who already has a smartphone will definitely keep unlimited data on that phone after the switch. It appears that you will be able to keep that plan if you upgrade, but there's no telling what you can upgrade from and to. All I know is that current 4g phone owners like myself will likely be able to upgrade to another LTE phone in 2 years without losing that unlimited option.
Then again, they haven't said for sure that "upgrade" actually means they give you the 2-year discount. It'd be ridiculous for that to not be the case, considering the average customer isn't likely to be aware of the full-retail price of phones and how the whole price subsidy thing works. But I wouldn't rule out the possibility yet...