I bought my '06 WRX Ltd in Aspen White on October 1. I called my dealer and told him I wanted Aspen White with the Desert Beige interior 2 days prior. They located one in a matter of hours and got it in the next day. The day after, I came in and filled out the paperwork and drove off.
I keep hearing the Aspen White is hard to get. Maybe I was lucky?
If you want Aspen White, be patient and get it. Have a few dealers try to find you one. If you order one, it shouldn't take more than 2 months depending on what time of month you place the order.