Waz up! with writing standards?

"Mrs. davis! What is teh homewokr!!!!???""
This is a standard line from a fourteen-year-old English student over AIM.
I support playing with words-- coining them and mushing them together in new ways earns gold stars in my class. However, these quoted lines don't scream 'creative license' to me: they shout sloppy.

A few months ago I was told that the reason students don't respond to my emails about homework is because they rarely check it. "They aren't into email. They are into Facebook and AIM," explained my tech-savvy colleague. Hoping for more frequent communication between teacher and student, I signed up for America's Instant Messenger, made a screen-name, and divulged it to my students.

"YOU have AIM!" they gasped in class; first worried about the overlap of boring school with their social lives, then excited that a teacher wants to be part of their lives.
"THAT is SO COOL!" they squealed, typing away and watching as their comments appeared on my screen projected onto the overhead.

"How r u??" they wrote.
"what r u doing???" they asked.
"I'm questioning your ability to switch between formal and informal writing," I wanted to respond.
Instead I just said "hi."

On the one hand I find it flattering that they are comfortable enough with me to let down their guard. Less stuffy, more open communication blossomed from our on-line banter. They work harder for teachers they respect and relate to, so why not make it easier to relate? However, I distain such extravagance as multiple exclamation points (doesn't one get the job done?) and with automatic spell-checker at their fingertips it seems reasonable to expect written communication to remain formal.

"What iss the dilly 4 tomorrow!!!!!?"

But how can I judge them when I myself slip into this tone with my friends on gchat? The reality is that their sloppyness results from an attempt at speed. It takes longer to make the proper letters into caps and stick a comma in the right spot. Should I, as their teacher, enforce this extra 'umph' or let it slide in our informal communication?