I don't know how long the Unit Conversion plugin has existed for Qucksilver, but I just found it the other day in yet another one of those 'QS spelunking' adventures that many of its users sometimes embark on (I highly recommend them). They're almost always a good time for Quicksilver fans, as one can usually find something new and incredibly useful on just such an adventure.
This particular Unit Conversion plugin/module allows a user to type the first half of a conversion in the first pane, tab over to chose the 'Convert to Units...' action, then tab to the third pane to type in the final unit conversion type. Clicking the 'i' on this module once it's installed offers a few examples, such as:
- 1 mile Convert to Units... km
- 1 floz Convert to Units... cm3
- 32 ft/sec^2 Convert to Units... m/sec^2