When it breaks. I love my iPad, I'm sure the ipad 2 will be an awesome device, but I bought this ipad because I love the device, not because I needed something to tie me over until the next one.
These things aren't cheap, maybe if it is AMAZING I would consider upgrading, but not likely. Cameras, meh, my iPhone works. More ram? Well, again no biggie, I don't use 30 apps at once. An SD slot? Camera connetion kit hasn't exploded yet. Significantly higher res display? Sold. But I don't think the technology is there for that, however. The experience of not being able ti discern pixels on the iPhone 4 is incredible, 300 ppi on the ipad would make my head explode, but beyond that, from the rumors, I don't see anything significant, not enough to warrant being without this for a few months, or to spend more money on a new one, even if I sell this one.
$.02