I'm down around 215 or so, but I've yet to have any serious issue with my 29er wheels. And I've popped plenty of spokes on a set of 26" wheels.
It's my theory that they might actually be *more* durable than a 26" wheel. If you accept the theory that a spoke can stretch under load, there's more spoke there to stretch allowing the wheel to absorb bigger hits before something snaps.
The stock rims on my Kona Unit 2-9 are the very definition of bomb proof. I've literally hit a tree, bent the fork, flatted the tire and the wheel didn't even need a truing. The stock rims on my Kona Kahuna require a little more attention on the truing stand every now and then, but are holding up fine.
If you have a set of road wheels working for you, go for a similar build on your 29er wheels in terms of spokes and rim design. In fact, you might even be able to use the same rims if they're wide enough. With more tire wrapped around them, they should be even more durable.