How can I tell how much grahics memory is available?

I don't know of a way to ask the graphics card exactly how much memory it has available at a time, but this page presents some system information that might be of use anyhow:

http://unity3d.com/support/documentation/ScriptReference/SystemInfo.html

In particular the graphics memory size. Instead of asking the graphics card how much it has left, would it be possible to tally up the size of the textures as they are loaded and keep track of how much you've spent, and then stop when you are approximately full?

Does the WWW object return a header that lets you know the size of a texture about to be loaded?

Update after more searching:

Okay, I'm pretty confident you can't get the actual, total remaining figure. I'm basing my conclusion on this forum post, where someone asked if there was a way they could get at the data displayed in the Editor's stats window. Aras answered that you can, but only in an undocumented editor class, and this was in 2008, so...err... I think you're out of luck. :(

Even the editor stats window displays guesstimate figures; there are two numbers, and the VRAM usage is supposed to be something in between? If they can't even get this number for the editor, I seriously doubt we'll be able to access it in scripts.