My bookmark is to
http://m.gmail.com, which is the mobile site that Google publishes and suggests people use.
I did have the same thought though last week, and tried the straight
www.gmail.com address, even though I do know that that URL auto-detects browser type, with the same results. I tried it again this morning, just to be sure some problem hadn't been resolved.
I did also try a forced-HTML-version link that I'd found someone on Google's help pages and had no luck there either.
If I had to guess, this is indeed a selective and random problem with the Gmail servers. It's certainly possible that desktop and mobile access would use two different mail servers while my mail file itself is stored somewhere behind those. Unfortunately, Google's tech support is non-existent (particularly with Gmail technically still being in beta), and their responses are either canned or consist of "if it doesn't work for you, we're very sorry but too bad" and so an actual answer from the source won't happen.