I'm flying United this weekend... if I use the curb-side check in, am I expected to tip the skycap? How much?

Yes, a buck a bag. they will also check in the car seat there, just take the stroller to the gate with you and they will load that right from the jetway.

You dont have to tip, but it is customary. You are using a service that is supposed to save you time in a way. Keeps you from having to go inside. Think about it, these guys are outside in the sweltering heat inhaling exhaust fumes all day. They deserve a tip. Depending on how much luggage, I usually tip $5 if its just myself. People that use Skycaps and dont tip, should be forced to stand out there for a whole day and really know what its like to be a Skycap, who makes far less than a regular agent hourly wise and their tips are their lifeline, much like a server.

Traveling with a baby, take the stoller to the gate. If it is an infant carrier/travel system...take it all to the gate. If there are extra seats you can have your baby strapped in rather than in your lap if they are a lap child. If they are a lap child you will be required to provide the birthday of the child and possibly have to show verification. If you are traveling with them as a lap child, they will usually send you inside anyway because you need to have your record documented and they are not trained for entering the codes, usually they are just a contract service.

When you get to the gate, check in and have your stroller tagged for gate claim and ask for pre-boarding. People traveling with children are always welcome and encouraged to pre-board so that you dont hold up the boarding process breaking down your stoller or getting to your seat. If you need help, just let the agents know, and they will be more than happy to assist you on the aircraft and to your seat.

I didn't realize curb side check in still existed. I thought it was over after 9/11

Curb-side check-in will already cost you extra, so I definetly would not tip.

I usually give $5......for 1 bag or for 6 bags.