Matt Yglesias and Kevin Drum have been talking about this recently, but since I've been reading a ton of undergraduate writing lately I feel uniquely qualified to comment on this topic.

Somebody needs to do an empirical study of these types of homophone and phonetic errors, and then provide elementary school teachers with a list of commonly misused words whose correct spelling and usage they shall beat into their students' minds. The only reason I don't make these types of errors all the time is because the teachers at Ironia Elementary School were merciless in their pursuit of homophones -- and as far as I can tell, aggressive vaccination is the only thing that works.

Here is a list of My Current Top Annoyances to get that intrepid psychologist started:

loose / lose
definitely / defiantly
suppose / supposed
site / sight
write / right
principal / principle
clicks / cliques
affect / effect
weather / whether
to / too / two

And, of course, there's the Possessive Wonder Quadruplets:

it's / its
they're / their / there
you're / your
who's / whose

Did I miss any? When we're done with the homophones, then we'll get started in on some other stuff in the English language I don't like, like exclamation points. We've got a long road ahead.

[/grammar nazi]