You won't be quitting your day job, but it is a curious idea.
I'm one of those people who are on the fence about achievements. They're often fun little things that for the most part are just unlocked for doing things in the game that you'd do anyways. That being said I believe that in some cases that achievements become overbearing and that they detract from the game. However, I did suspect that it might have only been a matter of time before something like this happened.
The Destructoid article that I learned about this from sums up the system succinctly:
"Earning 3,000 to 9,999 Gamerscore will give you a "special gift" (with
an approximate retail value of $0.25) during your birthday month; 10,000
to 24,999 Gamerscore adds a 1% rebate on Xbox Live Marketplace
purchases every month; and 25,000+ Gamerscore bumps the rebate up to 2%."
Admittedly those aren't exactly huge rewards. I'm not overly familiar with Gamerscore, but I believe that the hard limit is said to be 1000 maximum Gamerscore per base game, with extra being introduced if DLC is added. This is assuming that you go out of your way to unlock every single achievement. At the same time most achievements are worth ten to thirty points and are rewarded for things that people don't really have to go out of their way to do.
Now, it is worth mentioning that this program is available to Xbox Live Gold subscribers, but this is something that has a lot of potential for future use (and if people aren't careful, abuse). A 1% or 2% discount along with a tiny gift certificate during your birthday month might not seem like much, but you have to keep in mind that this is giving players a reward for something that they'd be doing anyways. As long as your Gamerscore is high enough you automatically benefit from this; you don't have to unlock every achievement for every game you own, just enough to qualify.
This isn't a monetization of achievements just yet, but it certainly does put such a possibility on the table. At the moment there's not much harm to it, but this is a system that will have to be handled carefully. Things like hacking achievements becomes a greater concern when they're not just a glorified form of e-penis comparison. You're also never going to stop people from looking at things like guides, but getting help and advice has never been a huge deal.
What I'm worried about is if this becomes a bigger thing, then will achievements become more and more esoteric and difficult to get in the face of the benefits from getting them? It kind of opens up a whole new can of worms. I'm hoping that's not the case, and certainly you can just ignore them and play the game as much or as little as you want either way, but I think that without some checks and balances there could be some annoying stuff on the horizon.