I have a field, of Users type, in a form, that has a default value of tp!owner.

Mike- who do you want to be selected? Depending on your configuration, task owner may not suit your needs.

Have you tried tp!assignees or pp!initiator?

How about:
forum.appian.com/.../Appian_Scripting_Functions

Hi Alex, I think the fundamental issue is that functionality apparent in webforms in not there in Tempo forms. I've now opened a ticket on this. I've also noted another fundamental difference between the two forms of display in relation to the display of type User on a LOV. In webforms the firstname/lastname combination is displayed whereas in Tempo you just see the username - not very helpful to end-users.

You can also try "loggedinuser()"expression if it fits your needs.