i'm in canada and i have to wait 7 months for my A5 i'm on month 2 and still going strong lol. S5 i was told is quite a bit longer of a wait. Not sure if US wait is shorter it all depends how many that particular dealer gets every year and where you are in the system so i'd phone around to a couple if you can.
4 months, or 8+ months, depending on how you calculate it. I "ordered" my A5 last July long before they were able to enter US orders. The official order was placed in November and my car arrived mid-March, a bit earlier than expected.
I can speak for Checkered Flag Audi (http://audi.checkeredflag.com)...where I ordered mine, and one of the stores in the dealer group I work for.
All of our 2008's are spoken for, but a loaded non-S-Line white/black demo will be available on June 21st (silent auction is happening now). The next 3 S5 slots are taken, and the first 2 2009 A5 slots are taken (one is mine). It is looking like we're wide open for anything after mid-August. We have had 2 people back out on 1 A5 and 1 S5, so you never know what might pop-up.
I ordered mine at the beginning of April, but I also wanted a 2009 so it is looking like my car will be here in mid-August. I could have had a 2008 by now.