With smartphones in hand, today’s travelers are staying in the moment — and booking hotel rooms at the last minute for the same evening.

2012 may go down as the year of the same-day, tonight-only hotel booking via smartphones and tablets as a bevy of travel sites, including Hotel Tonight, Priceline, Travelocity, and Jetsetter, now feature tonight-only hotel deals, with promises of discounts ranging from 40% to 70%.

The outpouring of hotel deals starting tonight, bookable sometimes through a few taps of your mobile weapon of choice, comes as Expedia and Travelocity say up to 70% of their mobile bookings occur on the same day.

This is a procrastinator’s delight.

The proposition is that these companies claim they can deliver steep savings off published rates at the last minute so hoteliers can fill rooms that otherwise would go unoccupied, with their bed sheets undisturbed.

Are you really getting a deal?

Screen shot of Hotel Tonight’s iPhone app.

You should always try to figure out if a “deal” is really a deal, and sometimes it gets complicated. For example, Hotel Tonight’s apps were offering the Super 8 Park Slope Hotel in Brooklyn, N.Y. on October 10, the night we were testing, for a total rate of $166, including taxes and fees.

And, that looked like a bargain compared with the Expedia.com total rate of $182 for the same hotel.

However, the Expedia app was touting the Super 8 Park Slope in Brooklyn for a total rate of $164.86, a buck and change cheaper than through Hotel Tonight.

The term “tonight-only” is a bit of a misnomer. Actually some hotels let you book their rooms starting tonight, but with the option to extend the stay for a night or two at the same rate.

These bookings require prepayment, of course, and usually come with no room type guaranteed.

The various companies providing these same-day bookings do so in a variety of ways. Here are the major tonight-only providers:

Hotel Tonight

Founded in 2010, Hotel Tonight pioneered the concept. Its iPhone and Android apps cover about 45 cities in North America and five in the UK, promising discounts of up to 70% off published rates. Hotel Tonight usually presents only three or four hotels at a time in each city, or city section in the case of large metropolises, and you know the room rate and name of the hotel upfront.

One of the distinguishing aspects of Hotel Tonight is its ease of use. After using it the first time, you don’t have to reenter your name or credit card details, and, of course, there’s no need to enter a date because you are booking the property for the same day. You can literally complete the booking in a handful of taps.

Jetsetter

Known for its limited-duration flash sales of upscale properties, Jetsetter began October 10 offering “Jetsetter Now” same-day hotel deals as a feature in its Jetsetter iPhone app. Jetsetter Now pledges discounts of up to 60% off published rates in the rollout, which for now covers seven cities, namely New York, Las Vegas, Los Angeles, San Francisco, Miami, Boston and Chicago.

Jetsetter Now is a semi-opaque offering, however: you don’t know the identity of the hotel in advance, and instead can read about a South Beach hip hotel for $126 per tonight, before taxes and fees.

Jetsetter claims that each property has been visited by Jetsetter personnel and passed muster. You can optionally use the iPhone to take a photo of your credit card as part of the checkout process. You can also optionally view the reservation when you arrive at the property when using the Passbook feature in iOS 6, Jetsetter says.

The limited number of cities covered and the lack of transparency about the hotel name are drawbacks.

Priceline

With one of the largest, if not the largest tonight-only offering, Priceline’s Tonight-Only Deals on its iPhone, iPad and Android Hotel Negotiator apps are available in 150 markets, and promise discounts of up to 40% off published rates on Priceline.com.

With Priceline, you know the room rate and hotel name up-front, and can also view star ratings and amenities, everything from hairdryers to premium channels on the TV.

One disadvantage of Priceline’s Tonight-Only Deals is that you have to pick them out on a map or list within the app amidst published rate, and Name Your Own Price offers. And, unlike Hotel Tonight, it takes a lot of taps to go through the booking process.

Travelocity

Hotel Deals by lastminute.com, a unit of Travelocity, are available in 36 cities, and you can book them on the iPhone only for today, tomorrow or the following day. Travelocity pledges discounts on these deals of up to 55% off published rates. Although you know the rate of these Top Secret deals in advance, you don’t find out the hotel name until you finish booking it.

As with Hotel Tonight, the lastminute.com app is easy to use, requiring just a few taps to complete the booking. You can scan your credit card as part of the process, but unlike Hotel Tonight, where you just have to enter your credit card details once, with lastminute.com you have to enter the information each time.

As with Priceline’s Hotel Negotiator App, the Hotel Deals by lastminute.com app is a bit confusing because the Top Secret Deals are intertwined with published rate offers.