OK, I know you all will have some questions. I'll try to collect them, and their answers, here so people don't have to read every post.
If you ask a question that is already answered here, I may facepalm and/or use sarcasm.
DO I NEED TO RUN THIS APP?
No, you don't. It doesn't do anything at all after installing. In fact, I re-used another app that I had handy, so the message you see isn't even accurate!
WHY DOES THE MESSAGE IN THE APP TALK ABOUT INTEROP UNLOCK? CAN I USE THIS APP FOR INTEROP UNLOCK?
It talks about it because I reused in interop-unlock app (which works exactly the same way, but with a different file). This app will not interop-unlock your phone, or even allow you to do so. However, it doesn't need interop-unlock, either. Furthermore, why are you running the app? You don't need to; none of the instructions say to run it.
CAN I DELETE THE APP?
Yes. Once it's installed, you can delete it immediately if you want to.
HOW DO I GET MY OWN OEM'S MARKETPLACE BACK?
You can either install the corresponding XAP and reboot, or you can wait a day or so for the phone to recover on its own.
HOW CAN I GET BACK TO AN OEM MARKETPLACE THAT I ACCESSED BEFORE?
Remove the app (and any variant of it), if it's currently installed. Then, install it again. Then, reboot as before, and you should see the marketplace corresponding to whatever XAP you used.
WHY DON'T ALL APPS WORK?
Many apps will use native drivers to do their work. These drivers are specific to each OEM's firmware, and are baked into the ROM. Apps that don't need special permissions, like the Samsung Now app or LG's Look n Type, should work. Apps that have high permissions or do device-specific things, like a Network Profile app or HTC Sound Enhancer (if it ever appears again) won't work on other phones.
SOME OF THESE APPS HAVE ID_CAP_INTEROPSERVICES, CAN WE USE THEM FOR INTEROP UNLOCK?
No. The apps may appear to work (even on interop-locked phones) because they were installed from the Marketplace and have valid signatures. However, they won't actually be able to do anything useful, because the wrong drivers will be installed (see the previous question).
CAN YOU ADD A XAP FOR ANOTHER MARKETPLACE?
Sure, once the required configuration data is available. This can be tricky - for example, the values for HTC and Samsung are "HTC" and "Samsung", respectively, but the one for LG is actually "LGE" (LG Electronics). I don't know what they are for Toshiba/Fujitsu, and I'm not sure I have everything from Dell, and I don't have any others. Any help here would be appreciated!
CAN I ACCESS MORE THAN ONE OEM MARKETPLACE AT ONCE?
No, sorry. I tried to build support for multiple OEM markets, but it didn't work. The last one overwrites the previous.
WILL APPS KEEP WORKING AFTER THE MARKETPLACE REVERTS?
So far, yes. I don't know whether they'll get updates, although I suspect they will.
CAN SOMETHING BE DONE ABOUT APPS COMPLAINING THEY DON'T WORK ON MY PHONE?
Sadly, no (nothing safe). Most of the LG and Samsung apps that I've tried work just fine (interop aside) on my HTC phone. It helps that in Mango, a lot of formerly interop capabilities (like compass for LG ScanSearch) are now in the official APIs. Some apps will still check though, and there's nothing I can do about that.
CAN WE ACCESS MARKETPLACES FOR OTHER OPERATORS?
It's probably possible, but I haven't bothered to try yet. If there are interesting apps for carriers that work outside of that carrier's network, I'll give it a shot. It's a different file but the structure is very similar.
IS THERE A LIST OF APPS KNOWN TO WORK?
Courtesy of user JusThinK, as of 1 Feb 2012:
Originally Posted by JusThinK
Look n Type
SmartShare(previously know as Play To - Working fine on with my Samsung HDTV)