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.
What would happen to top level domain names like .com, .net etc ? Even they use non-latin characters before .com, .net etc "com", "net" is still Latin.
Not exactly. There are already IDN TLDs in existence.
More specifically, the "." is still a separator. Any component of the domain name can independently have xn-- and it will be interpreted as Punycode. Today you could register ujjvala.السعودية and it would be completely valid.