Sorry for the trouble here, Adrian - try entering this into iCal's "Subscribe..." dialog instead:
Code:
https://sync3.omnigroup.com/<myname>/OmniFocus-Reminders.ics
(With "<myname>" appropriately replaced with your account name, of course.)
That'll skip a step (redirecting from the "sync" address to the specific server your account is stored on) that can sometimes cause issues. It'll also match the name of the file that OmniFocus actually places on the server.