laser on/off control and dxf 2 gc for laser cutter

I am currently a very happy LinuxCNC user, having replaced all the electronics of my old Denford Microrouter with a simple stepper control board that is now controlled via the parallel port using LinuxCNC. Many thanks to all contributors to this excellent project.

I have just ordered a cheap A4 sized Chinese 40W laser cutter. I intend to do the same to this; discard the provided electronics and software, and fit a simple stepper controller board and use LinuxCNC again.
I don't yet fully understand:
1) What gcodes are best to turn the laser on and off. I have seen a few posts on this forum recommending different gcodes and different ways of doing it, but I have not yet found anyone who has actually done it. If you have already done this; please tell me how.
2) How to convert from 2D design (DXF files from VariCad or QCad) to gcode, with the correct laser on/off controls embedded. To further complicate this, I am a linux only (debian squeeze) user (no Microsoft thank you). Again, if anyone else is currently doing this, please let me know how.
Thanks in anticipation.

Thanks for your answers andypugh.

Just in case anyone stumbles across this thread in the future:
Supplied electronics and software discarded. Replaced by a simple TDA 3 axis stepper driver and LinuxCNC.
I use a simple shell script to convert from Inkscape SVG to DXF:I use dxf2gcode by Christian Kohloeffel to convert from DXF to gcode. I simple changed the following two lines in the postprocessor config file:to make it correctly turn my laser on and off.
Brilliant!

I'm a month or so behind fishface, so I was glad to see this topic. I'm building an 80W laser, with a 2' X 5' table. I have most of the parts, and my laser tube just cleared customs today and will probably be here tomorrow. I still need to buy the aluminum extrusion and hardware for the frame and the skins, and an exhaust fan, and some small stuff, but I have most of it and I'm about ready to start building. I bought the XY table and I'll be cutting it down a little so it'll go through a door without disassembly, but the rest of the design is completely custom.

I had done enough research about raster graphics to realize that it's a bit of a stretch for LinuxCNC, so I was proceeding with the hardware, figuring I'd cross the raster engraving bridge when I came to it. I don't do much raster engraving, and when I do, it's not dithered photographs. It's generally letters and numbers, and a solid monochrome logo. It occurred to me that I can probably define the beam width as the tool width, and do vector engraving if I can move the head fast enough to achieve a reasonable engraving depth instead of engraving too deeply. In my sparse graphics, it might even be faster than scanning the raster lines with the laser off 90% of the time.

I was very pleased to do more research on Inkscape last Friday night. I grew accustomed to using CorelDraw for my laser projects, and I liked it as a CAD replacement, mostly because I could design in the vector drawing program and then print straight to my friend's laser as a Windows printer. It looks like it won't be much more difficult to design in Inkscape and then generate a G code file, hopefully with vector engraving to avoid the hassles of trying to get LinuxCNC to synchronize the laser ON/OFF with the raster scanning.

Good luck with your build BruceLayne.
Here are a couple of things that you may find useful:
1) A modification I did to dxf2gcode
2) A thread I found about simulating raster graphics , but I haven't tried this myself yet.