Is it possible/usual for them to strip? I'm sure mine isn't making any difference no matter how much I turn it!

Check:

1. Your cable pinch bolt at the mech end is tight.
2. Your outer cable doesn't have a split in it.

I have lost count of the number of times I've been twisting away at the barrel adjuster, slackening off, tightening the pinch bolt, turning the adjuster, etc, etc, only to find the outer cable's got a split in it somewhere!

Yep they strip, CRC used to stock them but ended up asking in the classified's for one from a broken unit, as I couldn't locate a new one

oh and SRAM & shimano adjusters are not interchangable

Yes, both external and internal threads can strip, and if it's the internal one for the front shifter(or you have a Shadow mech) then you may have to resort to in-line adjusters as used by roadies.If it's for a cable brake, the adjuster usually has a split which can sometimes close up, giving the impression of a stripped thread.As mentioned above, they come in various threads, and a decent LBS will have a few in it's spares drawer if needed.

Are you sure it hasnt just unscrewed all the way ?

I'm just trying to set up the front mech and I'm finding that the adjuster seems to turn and turn without "adjusting" shit anything.

I'll check the outers and have another go. I just rode a 28km ride without granny or big ring. (which are actually the two you can best do without I suppose!) )