I was in Europe over spring break and used my iPhone and incurred no accidental charges. If you're planning on using the phone to call home I would recommend calling AT&T and activating the International Voice Roaming Plan. It's $5.99 a month (you can cancel when you get back home) and it reduces the cost of most calls from $4-$5 a minute down to $0.99 per minute. It really depends on the country you're in but I'm almost certain Switzerland is $0.99 and Serbia as well. You'll get fun little text messages from each of the countries you go into welcoming you and telling you the service provider in that country, it's pretty neat.
As far as Data, those prices are extremely expensive and I would just try and find WiFi hot spots and make sure that Data Roaming is turned OFF on your iPhone in settings.
I will be in Ireland and paris for ten days (with my iPhone)... can i get a si card or anything that allows me to make local calls while i am there? i.e. to call between dublin and cork, or in paris, or between dublin and paris?
I didn't worry about obtaining a SIM card until I arrived overseas. I just walked around to a few different phone shops (there should be plenty) and looked for the SIM plan that seemed to have the best deals. I chose one that had free incoming calls and started with 50 outgoing minutes. I think it was something like $30 which wasn't that bad. I would call home, tell them to call me back, and talk to free.
You might want something different so be sure to ask around and you can typically find one that suits your needs.