Scores are as follows, all latest stable releases: (edit: incorporating other responses)

    OSX stable builds:
    Chrome:   288  +13
    Opera:    258  +7
    FireFox:  255  +9 
    Safari:   228  +7

    OSX dev builds:

    Windows 7 stable builds:
    Opera:    258  +7
    FifeFox:  240  +9
    IE9:      130  +5
    
    Windows 7 dev builds:
    IE10 PP:  130  +5

    Linux stable builds:
    Chrome:   273  +13
    FF 3.6:   155  +4

    Linux dev builds:

I'm surprised that firefox scored as (relatively) poorly as it did, although I suspect lack of support for closed formats had something to do with that. Similarly, I'm a bit surprised at Safari's poor showing given Apple's insistence that "HTML5 is the Future".

I'd be interested to see similar tests on Windows/Linux, with ie9 in there. Also, it'd be interesting to see how the beta/dev builds rate.