So I was thinking of picking this up from Battle.net, the problem is that right now the only computer I have access to is my Macbook which while very nice is not the ideal gaming machine. So i was wondering if I download the Mac version right now can I also download a Windows version if I get a new PC? Or even another Mac version if I end up getting a more powerful Mac? I downloaded a Mac version of Warcraft 3 as I still had the key from when I originally bought the Windows version and was wondering if Starcraft 2 worked the same way.  Can anyone shed some light on this? I couldn't find anything on Battle.net about it so I figured I'd see if the Giantbomb community could help me.

Yeah, after you buy Starcraft 2 (in store or on bnet) you can download it for either OS as many times as you like.

 Think of it as paying to activate the game on your account. The client doesn't matter. You can install from disks, from the Blizzard download, or even from "other" sources, and just log in with your account that you bought the game on. You can even log in on another computer that has the client installed and it'll load in your custom hotkeys, campaign saves, and other things from Blizzard's cloud. So, basically, Windows/Mac doesn't matter because as long as the computer has SC2 installed, your account is what lets you play.
 
Edit: Since you mentioned Warcraft 3, I'll make this a bit clearer. In WC3, your CD key was tied to the installation of the game; the actual client. You made an online account that was not tied to your key, but only one person could be online at a time per CD key. In SC2, your key is tied to your account, and the client is free for anyone to download and install. Think of it like WoW. You don't need a key to install it, you just need an active account (not that there's a subscription fee for SC2). This means you only get one online account, but you don't need an "activated" client.

Sweet thanks guys looks like I'm gonna go buy ti.

Being away from PC games, it blew my mind that I didn't need the disc and could install it on different computers (last PC game I played was Warcraft 3).  As mentioned, you're basically paying for the account.

Yeah, what they all said. It is really awesome, and in my opinion probably the best form of PC DRM that has been tried so far. 

Keep in mind that if you do buy digitally, it'll take a couple of days where your order is "queued" meaning that Blizzard needs to verify payment until you can actually use it. It took my purchase about two days to get activated.

Yes i bought my copy from the blizzard store on day 1,  it works very well and i didn't have problems.