Found this thread while researching the issue, here's my configuration:
- OS X 10.8.3
- iTunes 11.0.2
- 2 user accounts, each with its own Apple ID
- My Apple ID has Match (bought as soon as Match was launched), the other ID does not
- Two computers are linked to my ID, both with Match enabled (the other machine is a Win 7 PC running iTunes 11.0.3.42)
I purchased two albums from iTunes today, directly on my iPhone. No problems, downloaded the songs as normal and was able to play them. When I got home, I attempted to download and play those tracks on iTunes in OS X, but got the "This computer is already associated with an Apple ID" error. Turns out, the other user account on this Mac received the same error the other day. So, my purchase today doesn't seem to be a cause, just the catalyst for me to receive the error when I attempted to play the content on my Mac.
I have only ever had one Apple ID, and have had it for several years without a problem. The only issue I've seen in the last year is my Win 7 machine occasionally losing its Match setting, so I have to re-connect it to the Match service. It doesn't lock my content, just makes me go through the process of "re-adding" the machine, then everything works again. Don't think this problem is related, because it's been going on for a year.
As an interesting side note, the Win 7 PC that also has my ID and Match turned on, won't sync with Match tonight. It seems stuck, just continually saying that it's "accessing iTunes Match". I can't download or play the tracks I bought today on my phone (download button is grayed out), or any other content that's associated with my Match account that isn't already on the Win 7 PC.