I agree that we're unlikely to see anything like this again, I really wonder what's going to replace my N900.

To update an N900's hardware back to bleeding edge status, it would just need an updated CPU (Qualcomm Snapdragon/Samsung Hummingbird/ARM A4), more RAM, a higher-res screen, digital compass and maybe a WiMAX adapter. Also I'm starting to see LiPo batteries mainstream consumer products these days, so that might be a possibility. I really hope to see an update to the hardware in the future.