I still haven't gotten a handle on what I'm going to run this summer. I was going to do a sequel to last year's game, but that fell through when it turned out one of the main players wouldn't be able to make it for most of it. I could still run it, but I'd rather do something new, and go back to it later when I've got another game between the old game and the new one.
A significant part of me very, very much would like to run 4e. Not unreservedly, but enough that it's probably what I'll end up doing.
My original plan was to run the preview module, Keep on the Shadowfell, and spin off from there. Maybe even go on to Thunderspire Labyrinth, if Shadowfell is any good. I've never actually run a published module before, and I've got an idea that I might learn something from it.
Of course, I'm also thinking about running a palette-swap on the system, fiddling with the flavor to make it more steampunk. A couple of my players have indicated an interest in the genre, and I'd have a fair few crazy ideas to mine for a setting.
My other major ideas for 4e are to go with (a heavily hacked version of) the new default settting, or make it metal. These ideas are not incompatible, nor are they in oppostion to the idea of starting off with a published module. Even the steampunk idea could be made to work with that; I'd just have to file the serial numbers off the adventure, too.
And then there's the non-4e ideas. I've still never run Eberron, despite it being on my list since it came out, but the two players who have the book are the two who aren't available this summer. I've had Iron Heroes for ages, but I'm more likely to loot it for parts for a 4e game, since if I'm going to run a relatively combat heavy game I'd rather try out the monster/encounter set-up the new edition promises. Encounter Critical is an option; though a few people have expressed desire for a "serious" game, it might work as a pre-4e mini-campaign if I decide not to start the game with the module.
The final factor is the sandbox/megadungeon/hexcrawl game I want to run in the somewhat near future. That's not likely to happen this summer, partially due to player interest, partially due to work requirements. More likely, I'll run something else over the summer and start working on the sandbox thing either during that or right after it ends.