I've dipped my toes into Google maps gplotter.
We've got a commercial product from AFD to do postcode to national grid mapping, but I also tried this hack, though it was slow.
I converted national grid to latitude and longitude as required by gplotter using a spreadsheet and a hand-written macro to do it on a list in a spreadsheet.
The results look good, but I can't show you them.
Edit: postcodes to grid reference data is now available for free. See my post about getting postcodes in a constituency for the source.