Well, it seems that with Patch 4, BSP now fully works for me
So I've included a Zip File with the .BSP file, and the .DBA and .DBPRO source to load it, along with the gun from the tutorials for effect. Basically if you've had problems with BSP before, you can download and try this and see if it works for you, I can say it works 100% on my PC (AMD1800 1GbRAM GF4MX64 WinXP)
This is a Quake III BSP taken from the Quake 3 Arena Demo, as the most people seem to have had sucess with Quake 3 format bsps. There are dome black areas, but I'm pretty sure these are special effects areas in the game, like animated bits etc. You can add these separately in DBP.
For those who don't know, the included pk3 file is like a zip file, it can be unzipped, edited and re-compressed with your own bsps and textures

(Personally I like CuteZip, its compatible with Zip, Pk3, RAR & GZip and only costs $19.99)
Anyway here's a screenie
and here's the downloads
If you already have the quake 3 demo:
http://www.mywestsomerset.com/q3bsp.zip - 2.5 Mb
If you don't
http://www.mywestsomerset.com/q3bspbig.zip - 11 Mb
(The bigger file contains all the standard textures and therefore means you don't need the original Quake3arena demo installed to run it)
lol Its kinda funny such a nice looking demo can be made in just 10 lines of code, I remember typing out about 100 lines to make a single sprite on my C64

Let me know if the mystical BSP format now works for you guys
Cheers, Sam / Kangaroo2