Socket 1155 boards are available already. Just search for P69 or H69.
SB CPU's are available from next monday, at least, here in The Netherlands. Hardly can imagine they wouldn't be available in the US then.
Prices here are around 330 euros. But for now only 2 shops are offering them.
I'd definitely go for the 2600K model. It's just a few bucks more than a regular 2600, but it has un unlocked multiplier so you can overclock easily.
It is said that you can overclock to 4.5GHz on stock air-cooling.
Later this year 6 and 8 core SB's will be released, both with hyperthreading. According to Intel they are supposed for "enthusiasts" so it's likely they will cost >$600 each.
Hyperthreading works pretty well for TG. I know Frank has an i7 920 and I believe he told me once it is at least 1.5x faster than without hyper-threading.
On a $1000 machine (for example) I think an extra $100 investment for a hyperthreaded CPU is worth it.
I'm a bit undecided what to do. I have a Q6600 now which still does the job, but with an SB hyperthreaded CPU I could have a 2.5-3x faster machine, or wait for the six-core and have an even faster machine, but also an even more empty bank-account.