I'm positive you void the warranty on the computer. I've checked with a total of 4 employees in 2 apple stores. Yes, they will notice that the drive is not an Apple drive if you have to send it in and you leave your upgrade in it. No, there is no ink tab or anything like that. HOWEVER, the MBP is not the easiest notebook to get into in the world (though it's not the hardest either), and if you've never had a notebook apart before you could leave evidence of you opening it (say, a bit of damage to the head of a screw).
Furthermore, there are a number of flexible circuit boards that are taped to the drive. If they have it open (which presumably they will, if it's sent in), and they notice that the tape has been disturbed (and chances are good that they will), guess what? No more warranty.
I mean, look at it this way; the MBP-like most notebooks-has a pretty poor track record for reliability. You can also see common issues here:
http://www.appledefects.com/wiki/index.php?title=MacBook_Pro Personally, I've had a logic board and a hard drive fail in one, a screen fail in another, and this one that I'm using now has backlight issues. With the poor reliability in mind, consider that you're trying to save all of $150. Also consider that a screen replacement or a logic board replacement, out of warranty, will cost you the better part of $1000-if not more.
I do not trust Apple's service department. At all. I've heard far too many stories of them causing a LOT of problems for customers. I've built more PCs than I can count... I've even built a number of Macs out of combinations of modified Apple and PC parts. The Dell laptop I've got has been apart 5 times in the past 6 months alone. The MBP is the only computer I own that I will not take apart until it is out of warranty. I understand your desire to save money, but in this case I think it is extremely ill-advised.