Nokia's got plenty of good reasons for discouraging overclocking:

1. It could increase warranty claims (additional CPU failures, even if rare, may not just be statistical noise)
2. Reduces the likeliness of users buying the next device that comes out

On the other hand, that the n900 is so open increases their sales and the number of dedicated supporters/hackers like us. In theory might also generate some sales from those of us ethical enough to buy a new one when/if we brick ours. I surely wouldn't make a warranty claim after frying mine.

They're just being responsible by warning us. And in the end, we're sliding down the bleeding edge, because that's our move.

Quite frankly I wish more companies would take their attitude:

Permit all, but warn loudly