Looks like they are out of them now, but picked up a Chinese 3500 from Handhelditems.com for 15.99 including shipping. They called it the "Premium Extended Battery with Back Cover (3500 mAh) (1GgHTCEvo4g3500mAh). I saw the deal from a link i think on engadget or gizmodo. Did a search and it looks like, at times, they have had the same battery and back even cheaper (12.79) also with free shipping
Shipped very quickly and so far working very well. after 16 hours was at 45% after charging it overnight in a separate charger. Back snaps on well, looks and feels nice, but a bit harder to get off than stock.
so, if interested an not in a rush, or just decide at some point to go the 3500 route, might be worth checking them out to see if have them back in stock.
UPDATE: Dang, I'm lovin' me this battery. Day 3 and I decided to let it let it go for a while to see how it performed. Plugged it back in when it got to 8% after 31 hours and change with my normal usage including a lot of internet browsing and Pulse reading, email, phone calls and several Angry Bird sessions for good measure.
Now as a disclaimer, I do use SetCPU (rooted) to throttle it back when the screen is off and Juice Defender to turn off data use automatically from 1am to 7am. Lastly, indoors I usually keep the screen at 50% since that's what I prefer, battery or no, but during this 31 hours I was outside quite a bit at 100% so very impressed.
Now of course is to see how well the thing holds up over time, but a VERY good first week!