Built-in CELL function doesn't support external references, but it should.
Another external ref bug. I should fix this for 3.5.
http://office.microsoft.com/en-us/excel-help/cell-HP005209008.aspx?CTT=5&origin=HP005204211 (Excel 2007)
http://docs.oasis-open.org/office/v1.2/cos01/OpenDocument-v1.2-cos01-part2.html#__RefHeading__1018348_715980110 (ODF 1.2)
Initially just COL and ROW.
More on the way.
Now officially fixed.
We intentionally don't support PROTECT, PREFIX and WIDTH with external references.