As more passengers have smartphones, the airlines have gotten, well, smarter about delivering their content to them. One sign of the times is the ability to do paperless mobile check-ins for your flights. Think about that for a moment: you don’t need anything other than your phone when you go to the airport. (You still need a photo ID, however.)

I must admit I was skeptical when I first tried it out, but at SFO flying on United, I was able to pass your security and get in my seat just fine. The process was a great conversation starter for the passengers around me that saw me walk through with just my phone. I did feel somewhat naked, akin to when I first started using electronic ticketing years ago and didn’t carry my paper ticket from the airlines. Those days seem so quaint now, thinking back when we used travel agents (remember them?) to issue airline tickets that they would have to put in the mail for us.

Here is how paperless mobile airline check-in works.

When you go to the airline Web site to check in the day before your flight, you are given an option to go paperless. Different airlines have implemented this differently, which is your first challenge. (I didn’t say that it was effortless, right?)

There are basically two different ways: The first is to have the boarding pass sent to your email address as an attachment or a link. If you have a phone with a Web browser, you can bring up your Webmail account and view the boarding pass that gets sent to you that way. United and Delta rely on this email technique.

The second way is to download an app for your phone. Southwest and American have iPhone apps that you can use to retrieve your boarding pass directly from within their apps, which seems like a better idea but then you have to collect these apps when you use these airlines. (And the AA iPhone app also comes with Sudoku, so you never have to worry about someone already using the game page in their in-flight magazine!)

The iPhone apps are nice because you can do more than just check-in: you can check your award mileage, not that you will ever get a chance unless you plan months ahead to actually use it these days; make new reservations and more. With Southwest, having the iPhone app is doubly nice because as frequent SW travelers know, you have to check in at exactly 24 hours prior to your flight if you want that coveted “A” pass, and you can set an alarm on your phone to remind you if you are so concerned. The downside is if you have a Blackberry or some other smartphone: then you are out of luck because there is no alternative.

Regardless of how you get the boarding pass, the next step is to scan the bar code on your phone’s screen at the special scanners that the airlines and TSA have installed. It should verify you and if you are supposed to be there for that particular flight. This means that as you move through the security barrier and then to the gate, you need to keep that particular page on your phone open and ready to show to the TSA and agent at the gate. I was impressed that the agents knew how to scroll around my phone and view all the data on the screen.

The second challenge is that not every airport and every boarding area has the scanners for your phone. Not all airlines at all checkpoints in a particular airport have them, of course to make things complicated. For example, at SFO, United is good but Delta hasn’t yet got their scanners. Some airlines will just tell you on their Web sites which airports have them; some have implemented it in their Web code when you check in.

So another step in technological progress for travelers. Now if someone could invent an app that would prevent the middle seat next to me to be booked by obese passengers, I would be very happy. Happy trails.