Two quick questions about the N810 - first is there a way to connect it to the internet without the presence of a Wi-Fi connection? My cell phone allows me to connect my laptop to the internet via the USB port. The feature is not even advertised, and the connection is slow (100kbps, about twice the speed of dialup), but it would be useful to have if on the road or something.

That brings up my 2nd questions - what is the OS of the N810, and how is the 3rd party software? Anything useful? Is there a programming tool?

1) Yes, you can connect via Bluetooth if your cell phone supports the DUN profile. Some phones only support PAN, which is not officially supported by Nokia but can be get to work with the N810.

2) The OS is based on Debian Linux. 3rd party software mainly consists of ports of popular Linux software, but there's also a lot of tablet-specific software. The best of all, it's almost all free.
There is a SDK available from Nokia, so yes, there's a programming tool.

Is there a USB connection? Does it support flash drives?

Yes, USB-OTG, so flash drives work. Cell-phone connection as you speak of, might work, or might not. It should work, but I'm unaware of anyone who's actually tested it.

So will any Linux software run on it? It looks like there is a Connection Manager for Linux, does the software have to be specifically written for the N810?

No, generic linux software will not "just work" on the n8x0. Depending on the application the effort needed is somewhere between recompile and total rewrite of the UI in order to get it working.

In another thread, tz1 provided some suggestions to help get someone moving in this direction.