I got a B&M Dynamo Toplight Flat Plus recently and am trying to hook it up. Everything's fine, except there's no indication on the unit, no instructions included in or on the packaging, for which electrical connector tab at the bottom of the unit is positive/current or negative/earth.

Does anyone know for sure which is which?

B&M part no. is 329ALK-02 and you can find the light on the B&M webiste here:
http://www.bumm.de/index-e.html

Follow the menu chain: Catalogue --> Rearlights --> TOPLIGHT Flat plus

There is a pdf available, which is the packaging that came with the light. Note the helpful instruction "Electrical connection via current cable to current contact, earth cable to earth contact." Which is great except there's no diagram and nothing printed or stamped on the unit indicating which is which...