Tipping was definitely customary for curbside check in to the tune of about $2 per bag. Now many airlines have instituted curbside check in fees so that has many people confused. With many airlines you are still supposed to tip $2 per bag, but I believe I say a sign at the US Airways curbside check in at CLT that said tipping was not allowed. In absence of a sign like that I'd tip or take my bags inside and check them.

You should tip airport restaurants the same as any other restaurant in the US (nothing for counter service, 15-20% for table service).

Airline personnel (aside from curbside agents) are not allowed to receive tips so don't tip the flight attendant.