If you're a lightweight backpacker, you probably use some kind of tablet or chemical to treat your water. After all, these are probably the lightest options. However, some simply can't get past the perceived risk associated without using a more conventional and robust filter, like a pump.
Unfortunately, at 10-20oz, pumps aren't light. With seals, filters, and mechanics, pumps run the real risk of breaking or clogging which would render the expensive little gizmo fairly useless. Yet, some stick with it. Well, if you were wondering where you could find the lightest pump, check out the Nano Technology 530 (www.sportsimportsltd.com/530gacamicew.html), sale - $74.95.
The Nano 530 is a ceramic filter pump which can filter down to 0.1 microns which is about half the size of other commercial filters. It is only 5.5" tall and weighs an amazing 3 ounces. Because the filter is ceramic, it can also be cleaned which extends its life.
For those that may be interested, reviews say it works, but plan on gaining little output following a lot of work due to the small size. Five minutes of pumping per liter may be expected.
If you want to read up on filtering options and the nasties in water, check out my 2009 entry at http://jolly-green-giant.blogspot.com/2009/03/lightweight-water-treatment-options.html.