I thought I'd give WP_Geo a test drive, so I installed it, picked a Page, added a marker and voila! It didn't work - I got all the Google zoom controls, copyright line etc but no map or marker.
Changing theme from Scherzo (HTML5-based) to Twenty Eleven brings in the map correctly.
However: if I look at the page using Scherzo in Safari on an iPad it displays as you'd expect. It only doesn't display the map in IE8 or FF15 on Windowx XP or IE9 and FF15 on Windows 7 (these being all the other combinations of browser & platform I have).
I'm assuming it's the themes issue seen elsewhere, but it's odd that the Safari/iPad combination works while the other don't.