Christine,
I would also like to re-iterate the following problems with the DatePicker in Google Docs spreadsheets:
- the DatePicker should not appear when a cell contains a formula - computed dates should not be changed by the user, so the DatePicker has no use for such cells.
- the DatePicker should appear when a cell does not contain a formula but is formatted as a date and still is blank.
It might worth it to consider to tie the DatePicker to the Data Validation feature, also because DataValidation does allow to set some limits to the date input; these could be reflected in the DatePicker, e.g. by limiting - in the DataPicker interface - the range of dates the user would be able to choose from.
Personally I would value it when the team would consider creating interactive functions like a DatePicker spreadsheet function (which would allow the user to dynamically set the limits and exceptions on the dates to choose from) and a ListPicker function (again with control by means of parameters) which would bring the much needed drop down list feature to Google Docs.
I envision such interactive functions as having a mini-gadget interface which is invoked when the user would select or edit a cell; the functions itself could be used in a formula like any other function, e.g.
=GoogleDatePicker( start_date ; end_date ; exception_dates_list )
and
=GoogleListPicker( item_list ; allow_not_listed items_input )