The colours don't come in until Week 50 builds - mid December, so you won't be able to spec this on new builds until November ish (going on a six week wait for a build slot)... I'd imagine there'll be new literature out next month taking these and the other 2009 spec changes into account
Dealers obviously don't want to put people off by saying Astro Black and Lightning Blue are being discontinued in three months, so will carry on taking orders as usual right up until the announcement is made
Mind you, most dealers probably don't even know this is happening
Dealers will not be able to process orders for the new Convertible (R57) until early next year, possibly early feb - this is when build slots will be going live on their system and specs can be submitted for build.
Horizon Blue will be available on the R56 from Week 50 build, so this will be available to order shortly, however Horizon blue obviously won't be one of the special convertible only colours as its coming in across the range this year.
Which means the colours we have seen on the web images may/may no be horizon blue