It also irritates me to see "your" and "you're" mixed up.
However, we have to accept that English is constantly changing, and also that many changes come from the United States.
On UK message boards "your", as short for "you are", is still unacceptable by those who treasure the English language. However, US message boards seem to be accepting "your" as a suitable abbreviation. I predict that it will eventually be accepted by US educators and, later, by the OED.
As an analogy, most Brits still refer to a TV "programme" but happily accept that PC software is a "program". I wonder how long it will be before we accept the US spelling universally?
Chris
PS: Slightly off-topic, but I do find that spelling errors can conjure up some interesting pictures in my mind. At the school I used to teach at, there was a filing cabinet drawer labelled "PE stationary". Now that really is a whole new concept in education ;-)