This feature has been available on the official Google Maps site for a while, but it wasn’t available to third party developers until three days ago .
Another feature Google released recently is the ability to encode long lines for more efficient rendering. For long routes, this means the annoying “this script is taking a long time, do you want to continue popup” shouldn’t come up again.
We haven’t incorporated this into the trip planner yet but will some time in the next week or so (it’s a bit more complicated than adding scroll wheel zooming). I wrote some code that does the line encoding; now I just need to set up the back end to encode lines before sending them to the Web.