Fotiman - 6:32 pm on Mar 23, 2010 (gmt 0)
Note, I think you mean Chrome 4.0 (there is no 4.1 yet). :)
Some good references might be the Yahoo Graded Browser Support page:
As LifeinAsia pointed out, checking your own site stats is also good practice. For example, I personally don't try to explicitly support Opera because I don't get any visitors using that browser (and also because I trust that by developing to Firefox/Chrome/etc. that Opera should behave correctly for the most part).
And as incrediBILL pointed out, smart phone usage is on the rise... again, check your site stats to see if users are visiting your site with smart phones to determine what level you want to support them.
IE6 support has been dropped by a number of major sites, though it still has a pretty large market share. I would expect to drop support for IE6 in the not-too-distant future.