I got a text from this number about testing an iPhone 5 a few days ago, and received another one from another number this evening. Obviously a scam and highly irritating.

Text message said: Apple is looking for iPhone 5 testers! The first 1000 users that go to http://celltestandkeep.com and enter code 7923 will get to test & keep a new iPhone 5

Received the message: Apple is looking for iPhone 5 testers! The first 1000 users that go to http://celltestandkeep.com and enter code 7923 will get to test & keep a new iPhone 5.

I think it was only by chance that the text was sent to me and not necessarily because someone knows my number.  Unfortunately, if that is not the case there is really nothing I can do.

If you get this text, don't respond to it by saying "stop" or anything because that just tells the sender that they indeed sent it to a valid number.

Text message said: Apple is looking for iPhone 5 testers! The first 1000 users that go to http://celltestandkeep.com and enter code 7923 will get to test & keep a new iPhone 5

This is bogus! Apple doesn't let random people test products like this. Don't click, it has to be a scam.
I'm curious how they got my number however.