I'm a bit torn on whether i want sell my iPad 2 for the 3 or skip it and wait. On one hand, if the iPad 3 has a retina screen as rumoured, the same screen size, and at the same price points as the current iPad 2 then I'd definitely sell my 2 and buy the 3. The only problem is I won't know this until Apple announces it. But then the resell value for my iPad 2 will go down. I can't sell it too much earlier than release because I use my iPad to read my textbooks for assignments. I guess time will tell
You could sell your iPad 2 just before Apple's media event for perphaps $400. If the iPad 3 turns out to be a disappointment, you can get a refurbished iPad 2 for a lower price (same quality, 1-year warranty, lower price)
or a brand new iPad 2 assuming Apple drops the iPad 2's price (just like it did with the first generation iPad).
So, I'd say sell your iPad 2 (16 GB?) for $400 (should be possible, especially if it is in a good condition).
Than there are three possibilities:
1) Apple introduces an amazing iPad 3, you upgrade. The upgrade-price is $99.
2) Apple introduces a disappointing iPad 3, you stay with iPad 2. iPad 2's price drops (just like with first generation iPad) with $100. You buy a brand new iPad 2
with warranty for just $399.
3) Apple introduces a disappointing iPad 3, you stay with iPad 2. iPad 2's price doesn't drop (unlikely not to drop). You buy a refurbished iPad 2 for $429. Loss: $29.
4) Apple introduces a disappointing iPad 3, you stay with iPad 2. iPad 2's price drops. iPad 2's refurbished price drops. You buy a refurbished iPad 2 for $329. Profit: $71.
To me, it seems like a no-brainer to sell your iPad 2 for $400. Only in case 3 you have to pay an additional $29 (but than you do get a one-year warranty) - and option 3 is very unlikely to happen. It's really very unlikely iPad 2 will not drop in price.
So if we forget about option 3, you will ALWAYS get a great deal: you pay an additional $99 for the iPad 3 which will most likely have stuff like the A6 processor and the retina display. Or you buy a brand new iPad 2 for the same price you sold your old iPad 2: you get a one-year warranty and your iPad is brand new. Or you buy a refurbished iPad 2 which has also dropped in price somewhere in the $300-range ($319?) - you earn $71, get an iPad that most likely looks better than your old iPad 2 and a one-year warranty.