I really hope they have smart combinatoric search. I want to be able to search flexible day, flexible airport, multi-leg world travel trips without making my own spreadsheet. Some services do flexible day, some do flexible airport, but I haven't found any that provide these with a multi-leg itinerary. I'm sure this is due to the computational complexity, but Google could handle it.
Both OAG and Innovata provide a dump of all possible flight schedules for the next few months (which includes multiple legs and airlines). It is trivial to add the multiple day/airport feature: I've had a vanilla mysql install (on a P4 with a 1gig ram - my dev machine, sadly) return exactly the sort of stuff you mentioned in less than 100ms, so I'm sure the computational complexity is pretty irrelevant at Google's scale.
A co-founder of the company I did the work for had a few contacts in both companies, so we were able to secure the data through them. It definitely isn't publicly available, but I imagine if you talked to a regional representative at either company, you could get them to generate a sample data dump for you (a gigantic CSV file, for example)
Kayak used to have a free API, but they shut it down "because of constly misuse". I'm not aware of any other service that's targeted at individual developers - most of these APIs are intended for huge companies with correspondingly deep pockets.
ITA's Matrix beta site (http://matrix.itasoftware.com) does both flexible day and flexible airport for multi-leg trips. To do multiple airports, type in the airport codes separated by commas: "lax,sfo".
Wow. This is going to be interesting. Adioso.com (YC) is an amazing site, Google are going to have to try pretty hard to beat them for Australia, considering what Adioso.com can do without buying out a massive company.
I wonder if it will have Google+ integration from day one. Planning vacations together, or making sure you're never in the same town as someone? Maybe timing your visit to coincide with a candidate's campaign, or "facilitating serendipity" with people who will be at your destination at the same time. With circles and sparks, this could get real interesting....
I tried Hipmunk for the first time last week. I really enjoyed searching flights by "agony". Unfortunately I learned the hard way that all Delta flights should be put at the very end of the list when sorted in such a way.
Actually google does hotel pricing already in maps. It's not super obvious, but do a search for "hotels in San francisco" and you should see some prices
Or, another failed startup. This is the scary road you tread when you're in a space that might raise the eyebrows of a company like Google. High risk, high reward. I wish them luck!
Why do you think Hipmunk uses GWT (by which I assume you mean Google Web Toolkit)? The source for www.hipmunk.com pulls in jquery, but doesn't have the history iframe or <module>.nocache.js reference that one would expect to see for a GWT application.