I too thought premium was required. The owners manuals from the 2004-5 timeframe do require it. However, when I found this BMW owners manual for a 2000 328i, it only says "recommended".
The manual is at: http://www.bmwsections.com/_files/3s/2000_manual_3_sedan.zip
The relevant text is:
Use unleaded gasoline only.
Recommended fuel: Premium with a minimum octane rating of 91 AKI.
BMW engines are equipped with knock sensors and will adapt automatically to
different octane ratings, provided that the minimum octane requirement
(87 AKI) is met.
Fuels with higher octane ratings will provide enhanced performance and lower
fuel consumption, while the use of fuels with lower octane ratings will have the
That last paragraph does imply that engine will run in a "detuned" mode on regular - leading to decreased mpg.