Siwan is pronounced See-oo-an according to http://www.bbc.co.uk/wales/colinandcumberland/south/littleblackbook/wordbank/introducing_yourself.shtml (listen to the pronunciation).
However SHOO-an could also be possible because sometime si makes a 'sh' sound i.e. Sion, Sian. The w makes an 'oo' sound so it could be SH-OO-an.
The first pronunciation is from South Wales so perhaps their is a difference between North and South.
This message was edited by the author on February 28, 2007 at 2:57:50 AM