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Unfortunately, this is a bit more complicated than it looks, and in fact using other form elements for attributes on customers (and products and most anything else) probably warrants its own blog post that hopefully I can address in the future. In brief, to create a select box for a customer attribute you need to use the source_model attribute of the attribute. Have a look at the way the attribute is handled for the is_anchor attribute on categories. You can use the same source model for any checkboxes that you want as well. The source file for the attribute is app/code/core/Mage/Eav/Model/Entity/Attribute/Source/Boolean.php. There are also customer specific source models in the app/code/core/Mage/Customer/Model/Customer/Attribute/Source/ and the app/code/core/Mage/Customer/Model/Entity/Address/Attribute/Source/ directories. You also might need to make some backend models so that the attributes display correctly in the backend. The backend models are displayed in the Backend directories 'next to' the Source directories.