I've been succesfully running AdUC812-based development board from Linux. I agree, there's 0 of official support, but there's enough user-created tools. I used asem-51 as the assembler ($MOD file taken from asm51 included in the set), my own port of 'download' ported from the MS DOS C source (on the CD too, email me and I could send you the ported version), I've heard reports of successfully using SDCC for '51 programming in C, besides, the basic simulator and asm51 work fine under WINE (the included download program doesn't though). The learning curve is steeper than in Windows, but once you set up your own development environment and write a good makefile that does the compilation and upload for you, using it is way more comfortable than any windows tool.