Voigtländer Ultron 40mm f/2 is available in Canon mount. Photozone review praises its build quality and dislikes its rough bokeh. There's also 20mm Color Skopar SL II, but it's almost a stop slower than what you asked for, f/3.5. Photozone review describes it as well-built, but optically a little soft and vignetted when wide open, decent from f/5.6 to f/11.
With an adapter, you could use Pentax Limited line lenses. I can personally vouch for the 40mm f/2.8. An even smaller (and cheaper, APS-C coverage) XS version of it was announced recently, but it could be awkwardly tiny to use with manual focusing on a Canon body. But that's probably somewhat true for any pancake lens. Missing any way to control aperture of an adapted lens is probably a more important factor steering away from this option.
Update: Canon now produces the EF 40mm f/2.8 STM pancake lens:
Features (quoted from Canon's product page) include:
- Portable pancake design
- Smooth, quiet STM focusing when shooting movies on compatible cameras
- Fast f/2.8 maximum aperture for low-light shooting
- Close focus to 0.3m
- Fast, quiet AF for pin-sharp results
Plenty of sample images can be found in this Flickr group.