When I looked into which country Heroku’s servers are located in, I found that currently they only run on Amazon EC2’s North America region (as of January 7, 2012).

Let me quote several information sources.

Heroku runs on Amazon EC2, but it doesn't run on the Tokyo region. Of course, if you don't mind the latency of hundreds of milliseconds from crossing the Pacific Ocean, there's no problem using Heroku's North America region from Japan, but smaller latency is always better. There would be cases where you need fewer workarounds like placing static files on CDN.

・Source: Rails Hub Information Bureau: When will Heroku come to Tokyo region?

There’s a document that graphs Amazon EC2 East Region’s Response Times.

HTTP Response Time \\ ms 0 to 300, (31%) 300 to 400, (15%) 400 to 500, (13%) 500 to 750, (20%) >750, (21%)

・Source: Document visualizing various cloud responses published by Marty Kagan of Cedexis

From this, we can see that response times from Japan are at most around 300ms.

It would be nice if Heroku supported Amazon EC2’s Tokyo region, but I think it can be used from Japan without much stress even in the current situation.

Other Reference Information

Heroku runs on Amazon Web Services in the US East Region, which means that transfers to the S3 US Standard Region are completely free.

Source: Heroku | Dev Center | Uploading Files to S3

After Amazon Cloud’s Large-Scale Failure, “Data Centers Will Fail Someday” Becomes a Premise for Service Design - Publickey

That’s all from the Gemba.