You can definitely do this. I'm a Canadian and for years I've had a US iTunes account for getting the free apps and some US-only apps. This weekend, I also bought an iPad, and in order to get some paid iPad apps, I simply bought an iTunes gift card.
I used iTunes, logged into my US itunes account, then redeemed the gift card.
Voila! I downloaded and paid for Keynote and am now running it on my iPad.
I can browse the US store from my Mac or from the iPad or iPhone/iTouch.
Back home, you simply use your iTunes program, log into the iTunes US store and shop away. You can then log out, and log back in with your country's iTunes account and continue to use it normally.
When you sync your iPad/iPhone/iPod, all accounts that have been logged into and downloaded apps from on that computer will sync to your device(s).
Here's how you do it:
1. Open a new account in the US iTunes store
2. Supply US address & zip code -- you can find any number of valid addresses in the US. Personally, I used a relative's address but you can use anything.
3. Supply an email address -- did you know you can simply add a "+US" to your existing email from most servers? Check to see if it works. eg. user@gmail.com can also be user+US@gmail.com. This allows you to keep the email addresses separate for logins yet all will come into the same address.
4. Once set up you can download free apps at will. Buy and redeem an iTunes card to purchase apps. iTunes will keep track of your credit.
5. You just need to log into the US iTunes store occasionally to download the updates (you'll still see the update badge showing some number if you've updated all your apps from your home, but there are US apps needing an update).