The mobile app does not work at the moment with the upgrades. However, there is a better solution.
While browsing on your phone, go to the bottom of the main forums page.
Bottom left of the site, you'll see a pull down menu. Click it and select "Default Mobile Style."
Yeah, thanks Croce. The mobile skin is pretty solid, IMO.
Here is content-team's response to the same question about the mobile app in another thread:
In vBulletin 4.2 the mobile plugin is built in, so in order to upgrade I had to remove the plugin.
There's an API key that's required in order for the app to access the site API, and uninstalling (obviously) removes that. This was something I guessed might happen, and it wasn't easy to test in development. I submitted a support ticket immediately after the upgrade to vBulletin support, hopefully they'll have a solution.
I'm guessing I'll just have to regenerate the key and rebuild the mobile apps and send an update to the store. Might be a little while before I find out though. Apologies for that one.