P.S. Even better:
In TIME dimension TIMEID property is useless on parent members.
If for 2006.Q1 we put X in the TIMEID property then:
PROMPT(RADIOBUTTON,%ICR%,"Incremental?",ZZZ,{"Yes","No"},{"2006.Q1","ZZZ"})
ZZZ - not existing TIME member 
*SELECT(%IC%,[TIMEID],TIME,[ID]=$ICR$)
will result for %IC%
ZZZ - empty
2006.Q1 - X
Vadim