Just posted on using C# and the new object model in InfoPath 2007 to retrieve the current user.   I have 3 other posts that deal with this issue as well.  Believe it or not, the product team has actually addressed this in with a no code solution in InfoPath 2007.  Follow these steps to see for yourself.

1. Create a new form by Designing a New Form Template
2. Choose blank form
3. Add a text box to the form (Field1)
4. Go to Tools | Form Options
5. Go to the Open Behavior section and click on Rules
6. Add Action to Set a Field's Value
7. Choose the field you added above as the field to populate
8. For the value, click on the Fx button and choose Insert Function
9. View All in the Categories drop down
10. At the bottom you'll see userName, pick it
11. The formula that gets dropped in is userName()
12. Preview the form, you'll see your UserName in field1

That, my friends, is progress!

Tim