You can switch your store location to US or create a new US iTunes account without providing a credit card number but you have to provide a valid US address. Since the address isn't used for anything you can just use any address. Many people have used the address of the White House, for example. I used my old address from when I lived in the US. I'm not sure if it makes a difference, but some people recommend using an address in a state that has no sales tax (such as Oregon).
However, as you don't have a valid US credit card, the only way to fund your account for buying apps, music, books etc is using iTunes Gift Cards for the US Store, which you'll have to buy on eBay, for slightly more than their face value.
I did this for a while (created a separate, gift card funded, US iTunes account) so that I could buy/rent movies and TV Shows on my AppleTV as neither of these were available in the Portuguese iTunes store (where I live). I found that the hassle of having to load up my iTunes account with gift cards bought on eBay just wasn't worth it in the end. The other problem was having to constantly switch between my Portuguese and US iTunes accounts both on iTunes and on AppleTV.
As a result I haven't used my US account for quite a while now. The Portuguese store now has movies, but still no TV Shows or iTunes Match which I'd really like to have but I've decided just to wait.
Also, the automatic downloads feature of iTunes (for apps etc) is limited to one iTunes account per 90 days.
As to whether you could change back, when iTunes Match becomes available in Sweden? I don't see any reason why not. However, you almost certainly would have to re-'match' or upload your music again.