If it's brand new website, it's not uncommon to see them break into the first page of SERPs depending on the key phrases / competition. I've personally seen it on the local level (map results) and they've typically fizzled off after a week or two tops. I want to say Matt Cutts did a webmaster video on this same topic but can't find it for some reason. So to sum it up, if it is a brand new website, it'll probably start falling back in a fairly short period of time.
SunnyG, is it in fact a brand new website? This happened to me with a local based search about a month ago. Company A (my client) was being outranked by company B (their competitor) and after doing some digging around, we found that company B was a brand new website so I set myself a reminder to check that exact search one week from the day we found it and at that point it completely dropped off.