Looks like I misunderstood the recalc functions in Excel.

Shift-F9 recalcs only the active sheet and _only_ the cells that it thinks need recalcing (Ie. if a cell has a dependancy on another cell whose value has changed).

F9 alone recalcs the whole workbook using the same logic as Shift-f9.

Ctrl-Alt-F9 recalcs the entire sheet, cell by cell with no exclusions.

Since Excel wasn't aware that the logic in my Add-in server had changed, it saw no need to recalc the cells that used one of it's functions. Once I hit Ctrl-Alt-F9, I got the results I was looking for.

Problem solved.

Hope this helps someone.

-Matt