Is this sold as an assembled programmer?
Not yet, it is only available as a kit
How hard is it to assemble?
There are very clear instructions availble in the "Make it!" link. Its a simple kit and should be fairly easy for anyone with proper tools even if its their first soldering project.
Does this work with linux?
Yes. I have tested it with linux (Ubuntu 7.04) and it didn't require anything strange so it should work with any distribution
I need help getting this working
Check the HELP! page
Does this work with the 8051-core (AT89) series chips?
The USBtinyISP design as-is only works with the AVR core chips (ATtiny/ATmega/etc). However Lucas Chiesa and his peers have done an excellent job porting this version to support 8051-core chips.