I'd very much agree with Mike from my own experience of trying to race with a HRM - race day adrenaline and the stress and excitement of actually "racing" against other runners, all seems to throw accuracy out of the window for me.
The other point for me is that even in fairly short races (and even more so over the longer distances) there will be patches where you simply feel good and your body tells you you can go faster, and for me if one is slavishly watching the HRM rather than monitoring what your body tells you from moment to moment, you're going to miss those good patches and not make the most of them.
Racing well is surely very much about an intangible "feel", and I can't help thinking that using an electronic aid to pacing isn't just inhibiting when you've so much else to be thinking about, it's also rather missing the point of doing it in the first place - listening to your body and getting the best out of yourself.
Or is this just too Luddite an attitude for words? s.
PS. I just got my new Polar S210 through the post this morning (what a great toy!), so come next week I'll probably have changed my tune completely.
PPS. best of luck with the 1/2M, Martin, however you decide to race it!