i have a similar problem. i have been trying to burn DVD's in both data disk and movie form, and they cannot be read on macs! i put it in on the PC and they are working fine. i am using an external LaCie burner, my internal burner refuses to work at all when burning.
the thing i dont get is that i am burning it with toast and disk utility, tried it a number of times all with the mac compatible disks (UDF is compatible isnt it???)