Whats illegal is taping them without their knowledge. They could potentially make a call to their lawyer or doctor after they park it and you could unintentionally record privileged information. At some point such systems will be so common you wont have to mention it anymore.
While true, taking personal calls on the job is generally considered very poor form (and many service-oriented places do not allow it, like restaurants), especially where a motor vehicle is being operated. And given most valet driving trips last under a minute, it doesn't seem unreasonable to hold the call until the driving is done and the valet has exited the vehicle.
So practically speaking, I'd consider that scenario a non-issue.
Anyhow, the easy solution is since most cars have an LCD display for navigation as well as in the instrument cluster, when the valet key is used (which limits the car to certain abilities anyways) the displays could all say "This vehicle is under audio and video surveillance" constantly. After all, the LCD in the instrument cluster typically shows information valid for driving and irrelevant otherwise (e.g., fuel efficiency, trip routes, etc, none of which are needed to go between the entrance and the parking lot), and the radio/navigation screen isn't needed (I would hope the valet knows how to get to the parking lot!, and they shouldn't be touching your radio anyways - perhaps even have it be off if the valet key is used?).
Add in a notice on the valet keyfob as well and I think all possibilities are covered. Bonus is that there's no tacky stickers or signs for normal driving.