[EDIT] I stand corrected. Apparently, there is an international (PCT) route to U.S. patents. http://www.wipo.int/pct/en/pct_strategies/the_third_way.pdf [PDF]
the “international route” of the PCT to U.S. patent issuance. [...] THIRD WAY IS BEST WAY This Third Way of using the PCT as a matter of domestic U.S. patenting strategy gains many advantages for the applicant to control the timing of search and examination of the U.S. application, if one begins with a Provisional Patent Application.
USPTO has a diagram here: http://www.uspto.gov/web/offices/pac/mpep/documents/1800_184...
Under their system, it the whole process (to what they call the national phase) can be 30months - it varies from country to country, but 30 seems to be common. http://www.wipo.int/pct/en/texts/pdf/time_limits.pdf
I didn't say you needed to file everywhere - in fact you can't. You get to nominate in the WIPO process which countries you want to execute with.
The international examiners also give a lot of good feedback as they have a lot of experience about the individual countries - e.g. This will have problems in EU, but is fine in the US.
So if you decide the US is enough, that's fine - if you decide EU, US, AU, JP, that's fine too.
This is one reason to keep patents provisional as long as possible - you get the investors to cover the capital outlay of executing the patent.
If you launch into the WIPO "International Patent" process, for an upfront cost (US$5k or so - varies a lot on circumstances) you can extend the process by quite a bit - up to 18 months iirc. My bus partner wrote a blog on this after we'd done rounds of meetings with attorneys : http://www.jodoro.com/2008/09/patent-discussions.html