The bit about the rat urine causing illness created a bit of a stir with some blog readers. Here are a couple of links that you might like to try to learn a little bit more. In the meantime, washing off the tops of cans before opening them or drinking from them makes sense to me - I just don't like the idea of my coke flavored with urine, of any kind!

snopes

This came from the About.com "Urban Legends" site:
Comments: If you compare the two different versions above, one of which began circulating in 2002 and the other three years later in 2005 (with variants in between), you will find them identical except for the following features:

• The first claims the woman became sick in Belgium, the second in North Texas.
• The first refers to the disease as "Leptospirosis," the second calls it "Leptospirose."
• The first claims a study in Spain showed that the tops of soda cans are "more contaminated than public toilets," the second says the study was done at "NYCU" (perhaps meaning NYU, or New York University).

Not to worry -- neither version is true. Though rat urine can transmit diseases that affect humans (if the rat itself is a carrier), it is not inherently toxic or rife with "deathly substances." There is no record in standard medical research databases of a study comparing the cleanliness of soda cans with that of public toilets. Soda cans are typically stored and shipped in shrink wrap or cardboard cases, so they are an unlikely vehicle for dried rat urine, contaminated or otherwise.

Leptospirosis is a real (though relatively rare) disease and it can be transmitted via rat urine or feces, but the only cases reported in Texas over the past several years affected the canine population only.

This rumor is very similar to, and may have been inspired by, another email flier warning of deadly diseases transmitted via rat urine/droppings on soda cans that began circulating in 1999.