Ok, to add to this... An update with some more information.
I now realise that *technically* I have two Apple IDs in use.
I have my original Apple ID, which I've had since the year dot (well, 2004 from iTunes' purchase history).
I now also have an @me.com Apple ID, which comes from MobileMe being updated into iCloud. This is the Apple ID I use for iCloud, both on the Mac and on my iPhone and iPad.
As far as I'm aware, you can run with a different ID for iCloud, without problems.
I also have an Air (running Lion) and that too has the @me.com ID for iCloud and my old/original ID for iTunes itself.
I have attempted the same Match/iCloud track stream/download and the Air does just fine, while the iMac makes the same "Your Mac is associated to a different Apple ID and you have to wait XX days till you can associate it with another ID." and then it brings up the Error: 11111.
(note: track in question was uploaded to Match from previously mentioned work iMac... in which case I'm assuming the alternative MobileMe Apple ID used on the work computer isn't the issue - if the Air has no problem downloading).
Thus, I'm starting to assume it's either that *computer* itself, or specifically that *User's* settings for iTunes. Thus, I guess my next step is to create a brand new User on that iMac, set up iCloud with it's own ID, set up iTumes with it's own ID and try to download via Match.
If it succeedes, it's just that original User Account. If it also fails, it's more likely the whole computer 😕
My nephew has his own User on my iMac and I believe he has his own Apple ID in his iTunes settings... but surely since it's a totally different User on the Mac, that shouldn't affect it?
I'm in email discussion with iTunes support over this.
m.