Here we go again. American Airlines initiated yet another airfare hike this past Monday of $10 roundtrip; apparently the airline was not deterred by the fact that last week’s  $20 airfare hike was ultimately cut in half.  

Meanwhile, Delta matched this latest hike Wednesday morning but upped the stakes by making it a laddered hike of $10, $14 and $20 roundtrip, depending on the route.

And it wasn’t long before United/Continental and US Airways matched the initial $10 roundtrip airfare hike as well.

What does this mean? Well, it’s pretty clear that demand hasn’t softened – at least, not enough to prevent airlines from testing new highs for base domestic ticket prices.

To recap: For those of you keeping score: this is the sixth airfare hike attempt in 2011 – so far, with no matching from low cost airlines. Here is the 2011 Timeline:

• Jan. 9 – $4 – $10 roundtrip
• Jan. 17 – $10 roundtrip
• Feb. 1 – $4 – $10 roundtrip
• Feb. 18 – $10 roundtrip
• Feb. 24 – $10 roundtrip (initially $20 roundtrip)
• Feb. 28 – $10 roundtrip

Find Deals Now: You’ll always find the best deals available at FareCompare.com; see our latest Deals Roundup which includes specials from Southwest, JetBlue, Virgin America and more.

Friday’s $10 roundtrip airfare hike attempt initiated by Southwest Airlines (the fourth broad-based domestic hike this year) was quickly matched over the weekend by all domestic airlines.

It also appears that legacy airlines are adamant about raising the domestic price point for business travelers who buy tickets at the last minute or squeeze into the last seats on oversold flights.

Yesterday afternoon (Monday, February 12), American Airlines initiated a hike of $20 to $60 roundtrip on “walk-up” fares – again, the kind of fares last minute business travelers buy with a same-day advance price that can approach $1000 roundtrip. Later that evening, it was matched by United/Continental, and today Delta and US Airways joined in. Note that this is on the heels of last week’s similar airfare hike attempt, that ultimately collapsed.

Interestingly, the pace of recent domestic airfare hikes almost exactly mirrors those which occurred in the same time period of late 2007 and early 2008 – when oil prices leapt into the mid $90’s per barrel. The big difference today, however, is that airline baggage fees and widespread peak travel surcharges hadn’t been a staple in the total amount travelers shelled out for air travel.

We will update with any significant changes if and when they occur.

Here’s the latest, and it’s not happy news: On Monday, the FareCompare proprietary airfare processing system noted significant increases in domestic airline ticket prices for both business and leisure passengers.

Business travelers are accustomed to paying a hefty premium for the “luxury” of booking travel at the last minute (inside 14 days of departure) on schedule-friendly nonstop flights while leaving the cheaper seats for leisure travelers who can book earlier.

Monday, Delta initiated a domestic business airfare hike – effective immediately – of between $40 and $120 roundtrip that was widely matched by other legacy airlines in the ensuing 24 hours. This hike on walkup and last minute departures follows on the heels of a similar hike last week on business travelers who typically pay more than $800 round-trip for their tickets.

More importantly for cost-conscious leisure travelers, Monday airlines also began putting “travel date” fences on their cheapest airfare so that discounted seats could only be purchased through mid June, thereby bumping the cheapest summer ticket base prices by as much as $250 roundtrip (see the examples below for New York departures):

On top of these “base” domestic price increases for summer departures, airlines also continue to file “peak travel” surcharges for summer departure dates of between $20 and $60 roundtrip – with the higher amounts on the busiest departure days of the week.

The question for consumers is whether prices will come down in the following months as airlines begin to more actively manage summer inventory – or, on the flip side, might airfare prices get worse?

We are nearing the historical sweet spot for summer travelers looking to score a deal on domestic tickets who shop in earnest about 3-and-a-half months before departure – the time frame when airlines tend to start offering up some cheaper seats in order gauge demand in comparison to their historical booking models.

It is likely that early bird shoppers will be shell-shocked when getting quotes for summer travel so waiting may be their only option. One thing that is always helpful is to sign up for FareCompare Airfare Alerts and if prices on the trips you’re interested come down, we’ll let you know in real time.

Meanwhile, with volatility of jet fuel prices and the reticence of airlines to increase capacity coupled with firming demand, it is not hard to imagine a summer of some the highest airline ticket prices in years.

We’ll keep you updated.