Jeff,

This isssue has been addressed in IFR Magazine a number of times. In a
nutshell, you cannot use the VFR GPS as a "primary" navaid. However,
you can go dead reckoning if you wish when IFR. So, if you want to put
something in remarks, that's fine. Once enroute, if you want to go
direct, ask for a radar vector of ___ degrees, direct to ____. You are
then cleared via radar vector (all of it has to be in a radar
environment) and you are using the GPS as a backup, which is perfectly
legal. The controller gets a benefit because you suggest the heading
and you get off of his or her screen faster because you are going
direct.

For filing it doesn't hurt to file radar vectors to your destination
and note the VFR GPS in remarks.

Check out back issues of IFR Magazine for a fuller discussion. Also
check AVweb as I recall that one of John Deakin's columns addressed
this matter.

All the best,
Rick