I know it's not *strictly* a bag, but look how gorgeous it is!
Designed by Marie Louise Gustafsson, the basket is easily detachable and has a strap so it may be carried while shopping (so it *is* a bag!). It costs $60 from the MoMA Store and is almost enough to inspire me to get my bike out of the shed... Almost.
Related: Two wheels good: Cyclobags | The Sukie laptop bag from SF Moma | Plastic jelly basket from Topshop