Hey Sean -
Since SSAS will be parsing/executing the MDX in your calculated member, vba!now() should be just fine in your expression. Have you run into problems trying to use it?
Hmm, honestly I hadn't tried it - I'm very new to MDX. It turns out you're right, the VBA function does work. If anyone's curious, this is my code that gives Yesterday as a member that you can drop on the filter card:
StrToMember("[All Account Time].&[" + VBAMDX.Format(VBAMDX.Now()-1, "yyyyMMdd") + "]")
This is under the [All Account Time] hierarchy.