How can I circumvent regional restrictions in Google's Play Store? I'm travelling to country X. I want to install an application which isn't available in my country, but is available in country X; but I prefer to download and install it in advance, while the data connection is still cheap.

Unfortunately, the Play Stores does not allow me to install this app. Is there any way to circumvent this limitation?
Try the Market Enabler app (download the APK and side-load it.)  It allows you to spoof your region, so that the Market thinks you're in the US.

As an alternative, you may want to see if this app is available via other distribution methods, such as from other reputable markets (e.g. Amazon Appstore, SlideMe.org, GetJar.com, etc.)  Also reaching out to the developer can sometimes help.