Yeah, this is a disappointment for Merge users, and I assume will be the same for MX users since Alltel doesn't advertise the mobile hotspot feature for either phone. I spoke to a friend about this yesterday. He's a long time Alltel employee and recently switched from a BB to the Merge. He is also enjoying it except for the fact the mobile hotspot is disabled.
He's sure Alltel has it setup for a code in their system so that they can enable it ota with a paid subscription. But he also doesn't know when the option will be available. I imagine he'll find out more info as soon as anyone so I'll post when I know something different.
I don't have either phone (yet) but another Merge user posted in AndroidCentral forums about how to access the Wi-Fi HotSpot feature; eventhough it doesn't really matter b/c it still won't work. You may already know how and it could have even been you who made the post, but here are the steps for others:
Go to Settings / Applications / Quick Launch.
Pick something easy to remember like Search + w or Search + h.
On the list of applications to choose, select Wi-Fi Hotspot.
Apparently, there are also other apps found here that aren't listed in the main menu, like a Garmin Navigator app for one.