Techie question re my cell phone

Guys, I think I asked about this a few weeks ago but didn't really get a conclusive answer. As you know, I travel a good deal abroad for work and fun. I went to the local phone shop and they said for 30 bucks they could "unblock" my Motoral V600 phone and thus make it so that I can use the phone in Europe as a "local" telephone simply by buying a phone card with service in Europe. My phone is GSM, it does work in Europe, blah blah blah. My only question is whether this will work, and whether I can somehow screw up my phone by unblocking it? (Also, do I void my warranty, or can I "reblock" it later?).

As I said, my phone works in Europe, to the tune of $1.50 a minute. I don't want to know about plans, I want to know what happens when I unblock this phone with the guy down the street - can I then simply buy a card in Europe, insert it into this phone, and it will work as a local phone, then when I come home I slip my American SIM card back in and everything is honky-dorey?

Thank you for our geek minute of the day.