I knew it was going to happen, eventually. I disagree with something on these forums!
First, as an MIT (Massachusetts Institute of Technology) graduate, this thread's acronyms have particular appeal for me.

Okay, now down to business. I fundamentally disagree with the notion that the Someday/Maybe list is a series of commitments that needs to be pruned. Honestly, I have no qualms whatsoever about letting that list grow. Sure, I review it every week, but it doesn't bother me one little teeny bit whenever I scan through it with no updates. I've had items on that list for over two years...and it's far and away my biggest list.
DA talks about cleaning the garage, and how when a person wants to clean the garage it shouts out at him/her every time they walk by it. It's taking up mental RAM. On the other hand, if you want to clean the garage - but it's not IMPORTANT to you, or you're not COMMITTED to it - it belongs on the Someday/Maybe list. Once on that list, the garage cleaning party is out of mental RAM; it's on a list that, by definition, is filled with things that you MIGHT do SOMEDAY; you've not committed to do anything more with it than review it once a week, and consider whether or not it's something you still want to do.
You've let yourself off the hook about it.
You're not committing to yourself through the Someday/Maybe list. It's the exact opposite. If you start removing things from the Someday/Maybe list because you haven't done them, but you still retain some desire to do them, you're throwing those items back into mental RAM.
BAD! BAD! BAD!
I think this discussion is an issue of perspective surrounding the Someday/Maybe list. If you view it as something that you're supposed to be acting on, then it's not a Someday/Maybe list. The list is your freebie, catch-all, every-little-crazy-thought list.
There are only three reasons something should come off of Someday/Maybe:
1. You've done it.
2. You no longer have any desire to do it, any time.
3. You've decided that you are going to do it, and it moves into the appropriate context.
* Tim Glinatsis descends from the soapbox.
PS: If you ever get a chance to listen to DA's "Getting Things Done - Fast!" seminar, do it. You won't be able to buy it anymore, but I've heard that there are certain locations on "the internet" who might have it available for download (I'm not a lawyer, so I can't comment on the legality of such locations). He does an absolutely brilliant job of explaining the Someday/Maybe....and just about every thing else under the GTD sun.