I am using Odoo V8 with Odoo-provided test data.
Software updated from GitHub early today (27-8-14)

Just to test the Import/Export facility I have exported Product Data (all 40 products).
I generated two files actually. One using the Import-Compatible Export option and another with the Export All Data.
One field that was there was the Unit of Measure.

I then tried to import the data back into the system.
Regardless of whether the unit of measure id (uom_id) is
product.product_uom_hour or 
Hour(s)

I always get the same message.

Unknown error during import: <class 'openerp.exceptions.ValidationError'>: ('ValidateError', u'Field(s) `uom_id` failed against a constraint: Error: The default Unit of Measure and the purchase Unit of Measure must be in the same category.') at row 2

Clearly I went into the problematic products and checked that the units are not only of the same category but they are the EXACT same unit: Hour(s).

Obviously the error comes up with the Services.

If we are talking product.product_uom_unit or Unit(s) there is no such a problem.