I did some actual experiments on an iPod and I can confirm a few facts.
There is indeed a 90-day rule, it comes in to effect whenever you download "content" from the iTunes Cloud. (all the words in bold are important): It is not just relevant for iTiunes Match.
- the lock will indeed then prevent you from downloading content already purchased from the cloud under a different AppleID for 90 days.
- however you can still transfer, during this 90 days, content from a different AppleID from iTunes on a computer by syncing, and then play it OK.
- you can also download NEW content under a different AppleID to the one your device is locked/associated too. You just can not re-download previously purchased content.
- any content already on your device before it become "locked" is unaffected and can be played (even if it has a different AppleID).
- you can still stream content purchased under a different AppleID previously where streaming is available!
- IMPORTANT! all this does NOT apply to Apps! It just applies to movies/tv/music. So even if you lock your device you can still happily switch Appleids for App downloading. This detail confused me for a long time as most places I Googled did not distinguish this.
I hope this helps some people as it confused me until I tested it out!
Why this mess? I put it down to arcane licensing restrictions by the film/tv industry.