QUOTE(maledictis @ Apr 9 2009, 05:33 PM)

Yep - as an accompanist, I always take a breath just as the soloist is about to come in - it's a useful cue for them

I've been taught to do the same when starting off a piece as a continuo player.
On the original question - I do exactly what oldnotes does in certain places in certain pieces. Quite apart from the excellent medical explanation about oxygen supply, I definitely think holding breath like this is a habit with bad effects, which I'm trying to get rid of. In line with Alexander thinking (but I think this makes sense even if you've never come across Alexander technique or it doesn't do anything for you), I find holding the breath makes everything else that's going on in me - neck muscles, shoulder muscles, wrist locked vs free, mental and emotional attitude - also
held.
It's strange but I think the instinct to hold my breath comes from a useful and good motivation - concentration and a wish not to disturb a perfect atmosphere built up in the music. (For me it's a particular problem in very sustained, dream-like passages with difficult legato). But the effect (for me) is fear, because the music is only being sustained by force - a kind of feeling of "I'm holding on with my fingernails, and if I let go I'll ruin it". Inevitably I end up losing the hold!
I'm having to take the pieces that make me hold my breath right back to basics, and do a lot of practice dedicated just to watching what I'm doing with my breath, to break the habit.