Submitted by John
As an infrequent backpacker I've used the Katadyn Hiker, which I believe is similar to the MSR product. The "conventional wisdom" I've been exposed to is that in the US filtering with Hiker (or similar products) is usually sufficient (unless of course you're getting water from the pond described above). Katadyn has a purification tablet that is recommended for use in 3rd world areas (to deal with viruses) in addition to filtering with the Hiker.
ED - John, I dug some additional information up on the Hiker. It uses a pleated figerglass charcoal cored filter capable of filtering up to 200 gallons of water down to the .3 micron level. The unit itself weighs 11oz, is, 6.5 inches tall, and in looking at the images on the Katadyn website, it looks like the unit folds up nicely (the Hiker has a top mounted "plunger" type pumping handle whereas the MSR unit has a "lever" type pivoting handle.) The hiker retails from Katadyn for $
In addition the Micropur MP1 purification tablets you're referring to are chlorine dioxide based and are EPA certified effective against viruses, bacteria and micro-organisms (1 tablet per quart of water.) Katadyn offers the MP1 tablets for $13.95 for a supply of 30 tablets.