You shouldn't need to log into anything to be able to view the page, but at any rate, as tomku points out, we are going to use G+ for classwork discussion, so it's either now or later.
You can view the course page via the link given in a sibling comment, but be aware that the course is apparently using Google+ for class discussion as well.
The choice of Racket is because it contains several points of interest for what we're trying to teach. For instance, we will discuss advanced control (continuations), contracts, the typed/untyped boundary, etc., all of which are best represented in Racket. So the language itself illustrates some of the concepts we're trying to teach, making them more concrete and easier to experiment with (we don't first have to build the feature before we can play with it).
The ability to represent terms as s-expressions is actually a non-reason, given that we will not use s-expressions to represent terms. We will instead represent them as instances of a datatype, as in just about any other language.
The course is on how programming languages work. In order for an interpreter/compiler to interpret an programming language it must first parse all of the statements into a parse tree. Lisp is rare among programming languages in that its syntax is its parse-tree, which makes it an incredibly useful teaching tool for how programming languages work.
I'd personally prefer a language with static typing and ADT support, like Haskell or any ML-like one. But Racket (PLT Scheme) would be my second choice - and it is quite popular in overseas (USA) PL research circles. Also, I find it one of the most elegant lisp dialects out there.
