It's weak in comparison to what other manufacturers are doing with their refreshes currently. Of course it's more powerful than a card from 2007 (duh). As far as an improvement over yesterday's models, well the 6490 is an improvement over the nVidia 330... barely (and I mean barely as in "not really significant at all).
How many machines do you see launching in 2011 with a 256MB GPU? I know it's traditional for Apple to not put the latest and greatest tech in their machines but usually they try to stay competitive.
Apple don't tend to look at what other laptops manufacturers are doing, didn't you get the memo?