I've been told it's possible to move the area affected by a sustained spell (like physical barrier, shadow etc...) so you could move it with you as you walk.

Can anyone point me at where the rules are for this, if they exist, as I can't seem to find them?

SR4A, p. 184: If an area-effect spell is sustained, the affected area may be moved with a Complex Action, as long as it remains within line of sight. Characters who “drop out” of the affected area are no longer affected by the spell; characters who are “enveloped” by the area must defend against the effects of the spell as appropriate.