PAXBoy - I have been reading the comments with interest since my original posting and appreciate the answers.
The GPS doesn't transmit though - it's a receiver. I've used it many times when flying myself around (I'm a PPL) and so far, I haven't fallen out of the sky or been unable to speak with ATC because of interference.
I have also looked at several airlines web sites and both Aer Lingus (previously mentioned) and British Airways mention hand held GPSs specifically, with a permission to use them stated.
BA - "Electronic games, iPods, iPaqs, small radios, tape and disc players, miniature television receivers, GPS receivers and similar entertainment equipment have negligible effect on aircraft systems and may be used during flight, but must be switched off during take-off, approach and landing."
... which seems to contradict an ealier post that BA prohibit their use.