I've played Portal and loved it and am really looking forward to P2, but I've never played HL2. I know these two series aren't openly connected(?), but would it be worth playing through HL2 + Epi 1 and 2 before Portal 2 comes out? Think their stories might connect?

ya why not play half life 2!

It's not really necessary, but why not play it?

i will personally guarantee there is no story connection. however, i cant advise not playing half life 2. so do that

Portal connects to Half-Life, but not really the other way around. 
There are easter eggs and references in the Portal games that will benefit Half-Life fans, and Portal plays a part in Half-Life 2: Episode 2 and whatever they make next (though somewhat tangentially), but I don't think you'll get much more out of Portal other than references to Black Mesa and the Borealis (a ship that had Portal technology on it that disappeared and is found in the Half-Life episodes). They've said they want Portal's plots to be fairly stand-alone, despite taking place in the same universe. I don't think you'll learn anything about the Combine (Half-Life 2's enemies) in Portal or anything like that.

Half-Life 2 is great, but not necessary to enjoy Portal.

TL;DR It's been confirmed there will at least be a very loose connection or two to the Half-Life series in Portal 2.

Spoilers for some Portal 2 easter eggs, but not for the plot of either game:

There are several references in Portal to things in the Half-Life universe. 

" i will personally guarantee there is no story connection. "

" i will personally guarantee there is no story connection. however, i cant advise not playing half life 2. so do that "

Probably a reference like in the first Portal. Nothing overt or necessary.

Half-Life 2 and episodes would be a great way to find out why it's frustrating why Episode 3 has no date announced.

" @CoheedFavorHouse said:

" i will personally guarantee there is no story connection. "

same universe. "

" i will personally guarantee there is no story connection. however, i cant advise not playing half life 2. so do that "

How did you live so long without playing half-life?

Sure the stories might only be somewhat related but they are master pieces of their times.

Play em all I say you still have 9 days lol

You should go ahead and play Half-Life 2, but there is no need to do it before playing Portal 2.

 @Gearhead: i'd say they're pretty majorly connected. aperture science and black mesa were labs competing for grant money, leading to creation of both half life 1's events and portal's.

" How did you live so long without playing half-life?"

It's not going to do any harm checking it out. 

No. But you should still play it. It's excellent.

episode 2 is the most enjoyable. valve learned from the few mistakes they made in hl2 + ep1, but don't let that put you off. they're still amazing.

Play it, all the half life games are good, but Episode 2 is all the half life'iness you want with none of the filler.  Portal doesn't really directly acknowledge it though and i doubt 2 will. 

The connection is strictly casual, and I bet they never try to tie Portal to Halflife more than they have now. HL2 like I see youv'e heard(read) is excellent in it's own right. Give yourself the time to allocate for that rather than jamming in a non-existant precursor.

If you would like to see more of that 'universe,' why not play Half-Life 2? You're looking at a fair bit more combat, but it's still a wonderful, engaging first-person experience (an FPE, as opposed to an FPS).

" I've never played HL2."

Half Life 2 is a great game. That doesn't mean that aspects of it have aged well (The two overly-long vehicle segments come to mind), but it's still an important one to play (in regards to video games in general) if you have 10-15 hours to spare.

Half Life 2 is just worth playing.

 Yes, absolutely play it. It's an excellent series that's not difficult at all, with the exception of a couple parts in EP2. I don't necessarily think you need to play Half Life 1 though, but it couldn't hurt just for the sake of completion. I couldn't ever get back into HL1 to finish it either. I got stuck once in 1999 and never felt like getting past it.

I'm still mixed on the episodes of HL2 (as I got to the end & never finished plus not wanting to just console cheat the way through it), but the original HL2 game is quite well done & highly enjoyable. There's not too much to tie it to Portal1 but HL2 is definitely one game to check out & highly recommended. If it didn't have the gravity gun then I think that HL2 might be less entertaining for me.

As someone who first played HL2 last year, I can say it has lost much of its appeal. Like most older games it's not that great if you don't have any nostalgia for it.