I was thinking much the same - the Gmail backend surely has scaling issues, but the frontend should be relatively simple. Afterall, you can still use the basic html view if you want, the ajax view is just some icing on the cake; it can't be that resource hungry unless there are bugs.
There are always bugs in complex applications (especially when JS makes it this easy) and people leave their Gmail tab open for weeks: not an auspicious combination.
Oh yes, I wasn't trying to criticize that. Every sufficiently large program will have bugs. It was just that the title implied this was a scaling issue, rather than a debugging issue.
It seems to me that they're referring more to the size of the application. "GMail Scale" implies highly-interactive single page web applications with a lot of functionality, to me.
