Validation Transaction Column Field (Date)

Hi all,

We have a 'Transaction Column Field' on the item line the Quote and we want to add a SQL formula to the date field.
I hope somebody have a good solution for this without deploying a separate script to it.

We would like to edit teh field and go to > Validation & Defaulting > Default Value (Formula)
[LIST=1]
[*]Date needs to be equal or greater then today (Validation)
[*]When field is empty we want to leave it empty
[/LIST]

PS: I tried to use:
CASE WHEN ({custcol_date_requested} = {today}) THEN {today} ELSE NULL END


Thanks in advance