Check out this mask from Respro. I started searching for a respirator after I read that article about diesel fumes causing heart problems. This product seems to be ideal for city commuting...on paper anyway. I've ordered one and will test it out for myself.

The Respro Sportsta mask combines HEPA-type filtration performance associated with sub-micron pollutants such as pollen and other irritant dusts, with a lightweight Neotex aerated mask. The Respro mask offers maximum ventilation without sacrificing filtration and to maximize rider performance, state of the art Powa valves are a standard feature and a must for all who ride to their limits.
The Respro mask conforms to European Standard EN149FFP1(S) for solid aerosol particles and is usable in all outdoor sports pursuits requiring respiratory protection.

Usable for sports activities, pollution avoidance, and as a cold weather mask
Filters out sub-micron particulates, including:
pollen dust
irritant dusts
clay dust
black smoke
exhaust emissions
diesel particulates
Rapid airflow valve system offers low breathing resistance
Velcro fastening fits without fuss and is fully adjustable
Made from neoprene - durable and washable
Replaceable filter lasts approximately 150 hours

http://www.allergybegone.com/confacmaslar.html