Hi Nat13,
Whilst only you will know what to do, from having been in a situation where a so called 'friend' tried to sabotage my relationship I have three points to make after reading your story.
1. The myspace comment was on a public website so it was not done behind your back. This would suggest it was a joke/bit of fun?
2. I guard my privacy fiercely, I am very protective of my mobile as it contains private convos between me and my friends. I am the same with my emails and my other half knows not to go near them. This is not because they contain anything sordid, but my mum used to rake through my stuff when I was a teenager and laugh at my diary etc so I am quite paranoid about it now.
3. Beware of friends making sweeping statements about his fidelity. Unless they have hard evidence or can back it up, they really shouldnt be saying anything.
Only you will know what to do, why dont you ask about the mobiles? 5 weeks is early days, you should be having fun at this point and enjoying getting to know each other. One thing I have learned in life is that relationships are doomed without trust, be it friends, boyfriends or family.