If I start a new game in ME2 with my original character, I'll still be able to import my first save/playthrough into ME3 right?  My new save game won't save over my old one? 
Just wanted to make sure.

That's how it worked with the first mass effect in mass effect 2, also because I can tell the future on a game still on the storyboard, yes you'll be able to do.
 
Da da do do mmm just dance!!

It saves it under a separate "career," so you'll be safe so long as you don't delete the old saves manually.
 
I'd speculate that Mass Effect 3 might let you import a specific save file though, since the game allows you to continue after you've finished the main story, and tells you to go out and complete side-quests.

"  I'd speculate that Mass Effect 3 might let you import a specific save file though, since the game allows you to continue after you've finished the main story, and tells you to go out and complete side-quests. "