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.
My Setup: Crashplan, OS X 10.8.2, iTunes 10.7 (21)
I am unable to replicate the same issue on my system, and haven't noticed any excess changes or uploads to crashplan on my filesystem after playing music. To be sure it wasn't happening in the background, I ran a checksum on one of my audio files, played the complete song in iTunes, and re-ran the checksum. If something in the file itself had changed, the checksum would be different.
Here are the results:
Big-iMac:Abbey Road dave$ openssl sha1 17\ Her\ Majesty.m4a
SHA1(17 Her Majesty.m4a)= 3aea5150d7b1d21846b6502bac7ccacca44ff0fa
Big-iMac:Abbey Road dave$ openssl sha1 17\ Her\ Majesty.m4a
SHA1(17 Her Majesty.m4a)= 3aea5150d7b1d21846b6502bac7ccacca44ff0fa
As you can see, the checksum matches the second time after playing the song through iTunes. Most of my collection is ripped into ALAC (m4a), so I thought I'd test a standard mp3 file as well:
Big-iMac:One More Night in New York City dave$ openssl sha1 2-21\ The\ End.mp3
SHA1(2-21 The End.mp3)= a223d6fb542bee5191794d0a74f971da19fb9afc
Big-iMac:One More Night in New York City dave$ openssl sha1 2-21\ The\ End.mp3
SHA1(2-21 The End.mp3)= a223d6fb542bee5191794d0a74f971da19fb9afc
This too produced the same checksum before and after playing the song in iTunes. All of the metadata about your library (play count, etc.) is stored in the iTunes Music Library.xml file, not in the mp3 itself so I wouldn't expect individual songs to change. However, it's quite possible that the iTunes Music Library file will need to be re-uploaded since it changes frequently.
Best Answer
I opened my iTunes account in the US in 2009, and then moved back to Norway in 2010. After asking customer support, I learned that I needed to use up (most of) my remaining balance before I could change country, and I lost the remaining balance when doing the switch. (This was no great hardship to me. I had to use gift cards to pay when I was in the US, because my only credit cards were issued in Norway, so the balance was never very high to begin with.)
Of course, that was two and a half years ago, and procedures may have changed since then.