My family originate from Wales, and it was certainly always done there - as someone has already said, the coin was put in the baby's hand the first time you see him/her, to symbolise the wish that they will never lack money in their life. My daughter was born in the London area - and it wasn't done by the local relatives there (but the Welsh ones did it !)

A similar thing is done (again by my Welsh relatives) if ever giving someone a purse as a present, by putting a couple of coins (don't need to be silver ones) into the purse.

Congratulations, Bagpuss, on the birth of your new baby. Here's a "virtual reality" silver coin for your baby, with love from me:   O

Very best wishes,
- jeanie