Spatial data modelling tools
You could try GRASS or QGIS., open-source GIS applications.  You might be able to get what you want from one of them.

EDIT: I see you're after a data modelling tool with support for spatial data types.  I can't see any specific reference with basic google-fu.  You may be stuck with building your database model using CREATE TABLE statements and then reverse engineering them into a modelling tool for documentation.

If you have Visio Professional, the database modelling feature of that has a passably good database reverse engineering feature that will work OK with anything that supports an ODBC driver.

Another option might be to get an extensible tool such as Sparx Enterprise Architect and frig the metamodel.  However, this might be more work than building your database by hand.