Bulb Mode

I have been told ( by someone with a Nikon), that prolonged use of bulb mode can damage "something".

Is this correct? I have an istDL and I may be taking shots of a few minutes, and maybe even 35 to 40 minutes. Will this damage the camera?

Only references I found on Google mentioned digital cameras with possible pixels going dead from heat buildup. But I'm not sure you'll want to leave your shutter open that long due to the noise ratio becoming quite high after a few minutes.
I've done five minutes on my old istD and noise was not too bad. Newer models are probably better. Never done 30+ minutes though.

Just be sure your batteries are good. You don't want them dying while writing to the card. That supposedly can damage the card.

If you're doing something like start trails, you might want to take several shorter exposures and "stack" them. If its required for absorbing that much light... I'm stuck.

It is for star trails. I've done a couple with my 35mm SLR and although I was concerned about reciprosity failure, they turned out OK.

Iwill have to dig out my stacking software, I've got a copy of "Registax"....somewhere

You've just jogged my mind about reciprosity failure. Over the last 20 or so years, I've taken various night sky photos on 35mm (Leonid meteors, Comet Hale-Bopp etc.) which have turned out OK.
The only thing I've found with digital cameras is that the noise level appears to be the only problem with long exposures.
Yes, you have to be careful about always using a fully-charged battery, but apart from that I don't believe that you can actually damage your camera.
If it could be damaged, surely the manufacturer would issue some sort of warning to say so.
I've found this link which is quite useful;
http://www.geofflawrence.com/photography_tutorial_reciprocity_failure.htm

Hope it helps.

for star trails reciprocity failure is actually helpful.

The stars are the subjects and are rather bright. Each point on the stars arc is exposed over the few seconds to a minute the star is in that position so reciprocity failure doesn't have any real impact. The problem with star trails is fogging of the sky in general which takes place over the whole length of the exposure. Reciprocity failure means that this is unlikely to be registered at its full intensity so you get a blacker sky than you otherwise would.

The important thing with star trails is either to print them yourself or use a lab which actually watches what the machine is doing rather than just letting it go nuts on auto, otherwise you'll end up with grey skys.

oh and in answer to the actual question

I've been using my K10D and *istDL2 for astro photography ever since I've had them and neither shows any signs of ill effects.

One thing to watch though is that if you use a DSLR in place of the eyepiece of a Newtonian telescope then you are opening the sensor to the elements for the duration of the exposure. It will get very dirty that way. Dad and I eventually managed to construct a filter holder out of an old Vivitar 4 element TC, and this offers some protection.

Thanks Mongoose.

I've got a 4 inch (F10) refractor.
If we ever get a clear night I'll have a play and post some here for your C&C.

Prolonged use of Nikon/Canon equipment does seem to have an adverse effect on some peoples brains....

One thing I have noticed with using the B setting on my K10D is that it takes the camera several minutes to write the image to the card after the shutter closes. Why is this? If I were to leave the shutter open for several hours as is required for some cave photos would this cause damage? would this, considering how long it is taking the camera to process images taken over just a few minutes, overload the processors inside the camera?

It's the noise reduction process. Long exposures can create quite a bit of noise so the camera does a double-take; the first with the shutter open (your normal exposure) and a second with the shutter closed (which records noise only), it then takes the information from the second shot away from the first to leave you with a "clean" picture. Both shots have to be of identical duration so conditions are replicated, hence if you take a 10 minute 14 second exposure then the camera will take a second ten minute 14 second exposure immediately before anything is written to the memory card.

If you don't like waiting (and can live with the noise or deal with it in PP) then it can be switched off in the Custom Menu - look for Long Exposure NR.