I actually posted the entry below as a comment to comments made by my peers (and one worthwhile comment made by one of my Advanced Placement students: You think she's ready for college or what?). I was glad to obtain the perspective of one sitting in the same seat I myself sat ten years ago. I decided to make it a blog, as I would like to continue this discussion and I know not everyone reads the comments made on previous blogs.
Thanks for your comments, all.
I maintain that speech and writing have inherent differences. The asides mentioned by Brittany that occur in spoken language are not "this generation"-specific: My friends and I had similar digressions and lost the focus of conversations which began without one.
The thing which makes each situation unique is that, when a person is speaking to another person in a one-on-one setting (as is most often the case when IMing), one person has an obligation to conform to societal norms and allow the other to speak. Even if he who is not speaking is thinking about something entirely different or wants to change the subject, more than likely he will wait to voice his opinion until the other person has finished talking. IMs remove -- or at least lessen -- the need to conform to societal norms prevalent in speaking situations. Though many IM systems now alert users when another user is typing, there is still nothing preventing the person on the other end from typing at the same time. As soon as a thought comes to mind, it goes down in the IM text box.
In addition, reading is a different cognitive task than listening; students' varying abilities to attend to listening tasks versus reading tasks is evidence of such. This must be taken into account when discussing where IMing falls on the Writing-Speaking spectrum.