No clue. The only time I ever get a menu to choose my Launcher is after I've installed a GoLauncher update from the market. I check the "Remeber" box and choose GoLauncher, and all is good until the next update.
Yes, my remember this choice will not stick. I go to manage application and it shows it as sticking since I have the option to clear default. But then after using a few apps, the default got clear again. I can usually tell when it will happen by pressing and holding the home button to bring up list of apps and task manager. In the list of apps, twlauncher will appear there and I know instantly that the default to use it as the launcher has been reset. I cleared data in media hub, clear data in launcher, and then eventually clear data in twlauncher and the setting will still not stick.
I don't care which launcher I choose, launcher or twlauncher. The default won't stick and I have to choose it when I press the home button after a while. That was SO.... annoying so I reset the phone after exhaustive search on "launcher and twlauncher" on three or four android forums. I tried to delete launcher or twlauncher prior to reset but of course no option to do so. I just need one, I don't care which one.
Lots of you won't have this problem but when it resurface, make sure you backup all the important data before resetting. Backing up using Astro File manager was pretty much useless and I found out I can't back up apps data, only the app itself unless I root my phone and use Titanium or ROM manager. This is the only thing I miss about Itune. It might take forever but I don't have to pair bluetooth or start Angry birds all over again.