I do have over 300 stacks of cobblestone stockpiled, which is why I would've liked being able to donate them since I don't really need them at this time. I just figured that I should stockpile them in case I ever decide to build a huge structure out of cobblestone.
I'm currently thinking about expanding my minecart network, having enough iron to produce about 3000 blocks of railroad, but since autocart hasn't been updated yet for the latest updates ...
I do like the threat of mobs in SP, but I also like the feeling of a community effort you get when building stuff in SMP*, but so far health hasn't been implemented in SMP yet, although it'll be done soon according to Notch.
It'll be hard to basically abandon my SP world, but I think I'll go back to SMP soon.
* I started a project to recreate Isengard before the SMP reset, but we never got to finish it. It was awesome what we accomplished in a short time. At some point, there were about a dozen miners working on my project. I miss that.