For as long as I can remember adsense.google.com was how I reached AdSense. Now perhaps on some obscure blog Google announced they were retiring this URL (they don't usually have the decency to email announcements to users...), but the real point here is why?
If Ramone and I have even spent 30 seconds wondering why the URL wasn't working, that means thousands of users were.
Google advice to webmasters everywhere is to use a 301 redirect.
It looks like they have now fixed it - but it should never have happened in the first place.