it's a bit of fun as far as I'm concerned. I love HL, played both the first and second many times, might even fire up the first one again on steam on Saturday. In my opinion, HL3 will definitely get made eventually - it will make loads of money, simple as that. I expect a response from Valve after Saturday, they would be fools not to. Not a response that tells us HL3 is being made, or will be made, but just a response to acknowledge the fans enthusiasm - and I'm sure the vast majority will be happy with that. My personal opinion is that they want to make the game when hardware has evolved to a certain level - what that level is only they know. They naturally don't want to get peoples enthusiasm up prematurely, and after the hacking incident with HL2 I'm sure they run a very tight ship, but I believe it's ticking over in the background waiting for the right time to go into full production.
This is just my opinion and I hope not be flamed like others in this thread for simply being a big fan of something.