1. Can the application be ported?
Does your Windows Mobile application use features, APIs, functionality, etc. that does not exist or is not available to developers on WP7?
In many ways, Windows Phone 7 provides a mch more restricted environment for running applications. If your application needs to use some functionality you won't be able to port it. Alternatively you may have to change the way it works.
2. Should the application be ported?
If you have an internal business application which you need to control distribution of and/or be able to release fast updates to it. You may want to wait until there are distribution options which support this.
If you need this level of control you should strongly consider staying on Windows Mobile for such "task focused" applications. Windows Mobile isn't dead. It will continue to be supported and enhanced for just such scenarios.
Microsoft are currently investigating ways of allowing distribution to only a specific group of people. This feature will hopefully be made available in the not to distant future so may address some issues. This is unlikely to allow bypassing the application approval/verification processes though. If you really need to be able to push out updates in a hurry this platofrm isn't for you. (yet?)