Would be interesting to see the research data for this, anything you can share or some links that go into more details?
Personally I think the market is already saturated with Text Editors that use white backgrounds. Benefits of using a darker backgrounds is it's much easier on the eyes (as mentioned by Steve) and I also think it helps Sublime to stand out from the rest. It gives a much more sophisticated feel to it - most TextMate screencasts I've watched are using a darker theme, so I think non-Mac Python, Ruby, etc devs will appreciate that Sublime is a text editor that supports darker themes well. Also, a nice syntax highlighting color on a darker background creates much more contrast, so great for screenshots and to create that good first impression.
Besides, the colorschemes can be easily changed anyway, but to cater for both, might be an idea to add some dark on light screenshots to the gallery in the future. But as Jon said, better focus this effort on the editor itself for the time being.
Just my 2cents