Not letting me buy it either.
After the iTunes Music Store DRM changes my copy of iTunes popped up and said "do you want to upgrade this stuff you already bought to iTunes plus". I looked at what it was listing, figured that it was pretty much all stuff I really liked and so pulled the trigger on the upgrade. All very seamless. I subsequently learned that if I'd only wanted to upgrade some of it I'd be totally out of luck - it's an all or nothing deal.
But then I noticed that it didn't include one album I've bought not on iTunes but on my iPod Touch. They've got it in iTunes Plus format, in fact it's on special offer for £3.95 at the moment. But there doesn't seem to be a way to upgrade it.
Unless I buy it again. But not only is that a rip-off, it's on the Amazon MP3 store for £3. Including four bonus tracks.
So that's the obvious way to upgrade. But I resent buying stuff twice. Sigh, my own stupid fault. I know how incredibly bad DRM is, but bought it anyway.
By the way, now the Amazon MP3 store is working in the UK, I gotta say wow. It's cheap, it is only very slightly less easy to use than iTunes, no DRM. The only reason iTunes is selling anything is that only the geeks know about Amazon MP3.
Except iTunes does have slightly more stuff, so I just bought something off there that I've been searching for in vain for years. Sigh again. But this one at lease I made sure was DRM-free before I started.