DS is 3.5 and has an imaginary friend called Wozza. Wozza first turned up about a year ago and always seemed to appear when DS was going through a 'challenging' 
phase. We would hear about what Wozza had said/done and DS would talk to him on the phone. Now DS talks to and plays with Wozza (although this mainly seems to involve DS bossing Wozza about it has to be said 
). It doesn't really worry or concern me although I do wonder if it's intensified because there's been a lot going on here recently (moving house, starting pre-school, me expecting DS2 in March). I just find it quite fascinating and wondered how common it is/what experiences other people have of DC's imaginary friends, how long they stick around for etc.