Iphone Issue

My iphone used to sync with my old Sony Vaio, anyway got myself a nice shiny new 24" iMac yesterday and synced up the iphone with it, all was well until i removed it from the dock and now the Iphone won't do anything, the unlock button and home button do nothing, the screen remains black.

I ejected the sim card and the iphone sprung to life, re inserted the sim and played for a while and all was well. Then relocked the iphone and back to not unlocking etc..

Any ideas

Managed to soft reset it (found out how on a post here) and it seems to be ok again, is this a problem that i need to be concerned with... i.e hardware about to go wrong etc??

Soft reset was done by holding down the top button and home button for a few seconds until the apple logo appeared and that was it reset.

Anyone know how i can sync to my new Imac but keep all my music, ringtones etc..... Just that its synced already with the new iMac but now when i try to change to manual it wants to lose all the stuff on it..

Anyway around this ?