As I've said before, I really don't expect that the initial GS2 on VZW will have LTE. Samsung wouldn't want to release another device with issues like the Fascinate had initially, and with the Charge being such a hit I would expect they would want it to work with LTE at least as well as that. Not to mention that they would absolutely kill its sales if they released the GS2 with LTE. They couldn't charge anymore than $299, and an LTE GS2 at the same price would make the Charge completely irrelevant, despite a pretty hefty investment from Samsung and VZW in bringing it to market, so I don't see them killing its sales that completely.
There's also the fact that Samsung likes to brag about how thin the GS2 is, and I doubt they'd want to make one until LTE radios are small enough to keep the device nice and thin. And don't forget that VZW still doesn't want to overload the LTE network, at least not until they have tiered pricing or a more robust network to lessen the load.
I expect the Function will come out soon as a 3g device, and then once the LTE network is more widespread and stable, and/or tiered pricing is in place to balance the load, LTE radios will be small and efficient enough for Samsung to make a GS2 revision with an LTE radio in place with little to no size or battery penalty. Then they can release it whenever they feel the sales of the Charge are dropping off.
Speaking of which, I don't see what's so disappointing about the Charge compared to the GS2? I have a Charge and I love it, the best LTE device by far. Maybe you don't like the form factor, but the screen and camera are the same as the GS2. Considering those are two of the features that everyone raves about the GS2 for, and ones you'll be dealing with frequently, I would expect it to provide a fairly similar experience. Granted, the GS2 will have much more powerful hardware, which is of course a huge advantage, but its really the main difference. The Charge handles pretty much anything you want to do just fine, and there are already a few good kernels and ROMs that can really speed it up. So until Android gets to the point where the single-core is really slowing down, and dual-core is a huge advantage, I don't see why the Charge is so far behind. Granted, I'd love a GS2 LTE as much as anybody, but I'm very happy with the Charge, and don't see how the experience will be all that different for some time to come, unless you plan on sticking with the stock TouchWiz launcher...