I think you want a repeat set to "Start Again After..."
For multiple projects, it would probably go something like this:
To rotate through several projects, you'd have 2 variables:
P = the number of projects you want to rotate through and
D = how frequently you want to work on something (1 means each day you'll work on one of these, 7 means one project per week, etc.)
Set the "Start again after" interval to P*D.
Set the Start date of the first project to D days from now, the second to 2*D, etc.
You may also find this thread relevant:
http://forums.omnigroup.com/showthread.php?t=27981