Here’s one way to gauge it: track keyword searches using Google’s cool Trends tool, available here. With this free widget you can enter one or a series of search terms and instantly get a sense of how often they are invoked over time.

For example, a simple request for the graph of searches on Google (GOOG) for “3g iphone” over the past 12 months yields the fever chart below (subscribers click here):

Note the gradual rise in interest as Steve Jobs’ June 9 keynote approaches, which is not surprising. What is surprising is the fall-off in “News reference volume” in the bottom graph, a decline that seems genuine and not an artifact of Google’s data collection methodology. (There is no similar fall-off in, for example, Barack Obama searches.) This suggests that, although interest continues to grow among the Google-searching public, the tech press may have developed a case of 3G fatigue.

Google Trends also shows you where these searches are coming from. Here’s that data for the chart above:

Check out the size of that Hong Kong bar! How is it that an island with less than 1/40 the population of the United States generates three times as many hits? [Correction: Google is showing something more like searches per capita; see here. Still, there seems to be a lot of interest in the 3G iPhone in Hong Kong.] Let’s zero in on Hong Kong’s 3G iPhone searches:

This shows a sharp rise in searches that began in the middle of May, even before last week’s announcement that Hutchison Telecommunications would be bringing the iPhone to Hong Kong and Macau.

Here’s one final chart to put things in perspective. It maps 3G iPhone searches against RIM’s (RIMM) BlackBerry and simple iPhone searches, without 3G.

Note that despite the recent uptick in searches for 3G iPhone, they don’t rise to the level of BlackBerry searches. Moreover, neither can come close to the buzz for the original iPhone, especially when the device was launched last June.

There’s lots of data to be gleaned by tracking Google Trends. If you find something particularly noteworthy or surprising, take a snapshot and post it in the comment stream.