Perhaps I wasn't totally clear in my original post though- my problem isn't really that I think my voice isn't good enough, it's simply that as soon as I sing through a microphone, I'm unable to sing in tune because I hear myself completely differently compared to singing without a mic (similiar to your first experiencies PonyOne ;) ).
Whereas when I sing the same song without a mic with, say, an acoustic guitar it sounds fine (to my own ears).
Dr_Simon: you mentioned the first trick is to "hear yourself properly". I understand this is what monitors are for? And that the definition of hearing yourself properly implies that the singer hears his own voice as if he wasn't singing through a PA system at all? I'm affraid I don't have any fancy equipment to my disposal.
I suppose what I'm really after is people who have experienced the same problem, to re-assure me it'll pass in time ;) . If you have had a similiar experience with "learning how to sing, *again*" please do share!
Cheers