When I tried my upgrade, I got an e-mail quickly, and was told "Your iTunes Plus music upgrade could not be successfully completed because there is a problem with your payment information." Now, I am *certain* there's no problem with my credit card info, so I try again. Same deal. I wait a few minutes, try a third time. This time my Upgrade has been "in process" all night with no subsequent e-mails and the upgrade page no longer shows me how much is available to upgrade; and anytime I click on the "iTunes Plus" link, I get a dialog box about how I'm currently processing.
FYI - I have a *very* large upgrade to run... it was just over $300 when I tried it and seemed to be steadily growing by the minute.
I should have been more precise. This initially happened to me. I got an e-mail within five minutes saying the same thing. My problem was that I had an out-of-date iTunes credit balance (probably from buying stuff on my iPhone). As soon as I logged into my iTunes account, it updated the balance, I clicked on the "Upgrade" button again, went to bed, and woke up this morning and still hadn't received the e-mail. I tried upgrading again and it worked this time. Apple's apparently been working over night because I had about $20 worth of new stuff to upgrade this morning (it was about $120 last night and $140 this morning). I expect when I get home from class tonight, I'll have even more.
I don't know if you have any iTunes credit, but try logging into your iTunes account. That solved it for me. I also have a relatively large upgrade to run and it's currently doing it while I'm at work.