I'm wondering if there's any performance impact for the people who get the "main" theme, and whether anyone has modified the Base theme to pop up a little explanation of why they've been sent to the alternative theme.
Yes - I am, but its not to Base, its to a slightly recoloured version of the standardold theme and I just included a little line of text in the header area to explain why they should be using a modern browser and to provide links to download the latest firefox and chrome.
I have not noticed any performance hits for any user in this for users of the main theme - and I can't actually tell you whether its being used by anyone (I hope its not!) but I didn't notice any performance hits when I tested it originally.
Standardold is classed as the Legacy theme for people to use when their only option of browser is IE6. You need to talk to Tim Hunt about this, I think he would be able to tell you.
Are you sure it's IE6 only? It would make sense for IE7 also since IE8 is the "standard" now.