I had a quick play this evening as I've used but not played with greasemonkey and google maps as yet.
So I put together a simple script that adds a small google map to the bottom of London OpenGuide pages that have coordinates on.
It "cheats" and inserts an iframe that loads the php based map page up from my site.
greasemonkey script can be got from here: http://www.cowfish.org.uk/maps/ogl.user.js
php src from here: http://www.cowfish.org.uk/maps/ogl.phps
greasemonkey can be found here: http://greasemonkey.mozdev.org/
google maps api can be found here: http://www.google.com/apis/maps/
and a screenshot of what I done did here: http://www.flickr.com/photos/cowfish/22442786/ (click all sizes for a full size version)
The google maps API access is being restricted by a key which is being generated from the URL that you want to use the API from. This is one reason why I went the PHP route, as I don't know what they are doing with the submitted URLs and in a fit of paranoia didn't want to send on the ogl URL. I also don't know whether I could dynamically load their API script from greasemonkey, but haven't got that far yet :)
Anyway, thought I'd share and see if anyone else has played with this kind of thing and has any tips.
billy