I havta tell you, the best I have seen was the Nokia "wrap" they sold for the n800. I used that and carried it in a pocket all the time, it protected the screen and made it easier to hold on to. However since you asked about the N810, I have been using the OEM case for that because:
1) one of the BIG advantages of the N810 over the N800 is how small and thin it is. putting a big wrap-around leather case with card holders and a big closure strap makes it SO large it no longer fits in a pocket, if I have to carry something around with me I might as well carry my Fujijtsu U810 (810 must be the universal small computer id number) which is in a case not much larger than the N810 cases I have seen.
2) anything else small enough to take the place of the OEM case will be essentially the same thing with the exception of costing more.

I wish nokia would make one of those magnetic wraps for the n810, yeah I know the slide up screen is an issue, but with the wrap, it is already out of the way when you slide it up.

-John