For the past six months, the pace of airfare sales has been torrid. And now this:

Delta/Northwest, along with US Airways, initiated a system-wide U.S. domestic airfare hike late last Wednesday – a hike of $20 roundtrip across the bulk of their respective route systems.

In the days that followed, this hike was widely matched by all legacy airlines. And on Friday, the low-cost carrier Southwest Airlines joined in, virtually assuring that this first major hike attempt of 2009 will stick.

U.S. domestic airlines have been searching for a bottom the past six months and it appears this hike, along with a slowdown in domestic airfare sales, is the first sign the bottom is near.

There has been grumbling in the past week from a few airlines that the industry has been excessively focused on domestic load factors (ostensibly to make up for the heavy shortfall of higher paying business travelers) to the detriment of the yield (dollars per seat).

Attention shoppers: The hike, and the recently announced capacity cuts – along with the prospect of higher fuel prices (oil has doubled to $70/barrel from its low in February) – do not bode well for consumers hoping that the wave of nonstop discounting will continue through midsummer into early fall.

More about the hike: It is an increase on base airfares and not a fuel surcharge; it should also be noted that there are still some U.S. domestic sale airfares in the marketplace which were spared the hike (but most of those sales are set to expire this week).

International note: We have also seen a dramatic slowing down in the scorching rate of international airfare discounts this past week – which could be a sign that the bottom may be near for those airlines with extensive international networks.