Usually Audi/VW starts building the new model year on calendar week 22 (last week), this also has been confirmed by reports to have happened this year as well. So a car built on week 26 definitely will be a 2009 model.
Trunk lining will be darker colour (black instead of grey). If your car is to have side assist then it will work from 30 km/h upwards (old was 60 km/h). (In the U.S. side assist is apparently part of the tech package starting from model year 2009, before it was a stand-alone option.) If your car has drive select, then the colour/type of the plastic controls for drive select have changed (matte instead of glossy).
Also other small visual changes, software updates and some minor upgrades to electronics (some rumours of improvements to the Audi bluetooth car phone, who knows) wouldn't be unheard of. We pretty much have to wait until cars start to hit new owners (next week for the earliest ones perhaps?) to know of such details.
Oh, and I almost forgot: AUX IN connection for all radios (and MMI/navi too) in the center console for connecting, say, an iPod (even without AMI).
There are also reports that emissions might be a bit lower for some engines in model year 2009.
There are also some new options for model year 2009 but those will probably not affect you unless you manage to make changes to your order with your dealer.
German list of some of the known changes for model year 2009 (not complete though, at least AUX IN is missing from this list but is confirmed by new pricelists), try babelfish.yahoo.com for translation:
Audi A5:
+ Option: S Sportsitz für S5 mit Alcantara/Leder-Kombination (mit S Prägung)
+ Option: S Sportsitz für A5 mit integrierten Kopfstützen (ohne S Prägung)
+ Option: Lederpaket, belederte Armauflagen in den Türverkleidungen vorn und den Seitenverkleidungen hinten
+ Gepäckauskleidung in Schwarz statt grau
+ Schalter Drive Select in schwarz matt statt schwarz glänzend
+ Side assist arbeitet schon am 30km/h (statt 60km/h)
+ Option: Fernlichtassistent mit Licht/Regensensor und Scheibenwischer-Intervallschaltung
+ Option: S line Exterieurpaket: inklusive verchromte Auspuffendrohrblende bei 4-Zylinder