Just a quick note on something that had me scratching my head for a couple of hours this afternoon. The task was to create a new custom field type that is based on the SPFieldUrl class. Although my fldtypes_*.xml file *looked* correct it wouldn't load. It turned out that the ParentType property value is both case-sensitive, and inconsistent. Whereas for a field that derives from SPFieldText you use set the property to "Text", if the parent type is an SPFieldUrl, you need to set the ParentType property to "URL" (all upper-case)!
So:
<FieldType>...<Field Name="ParentType">Url</Field>...</FieldType> is WRONG
<FieldType>...<Field Name="ParentType">URL</Field>...</FieldType> is RIGHT
Gah!