http://www.asaris.de/sites/walsh/Lists/WSSv3%20FAQ/DispForm.aspx?ID=691

Site Creation

SPSite mySite = SPControl.GetContextSite(this.Context); 
mySite.WebApplication.Sites.Add("sites/" + SiteName.Text,
SiteName.Text, SiteName.Text, Convert.ToUInt32(1033), "STS#0",
Administrator","Administrator", email@xxx.com);

Sub-Site Creation:

SPWeb myweb = SPControl.GetContextWeb(this.Context);
SPWebCollection mycol = myweb.Webs;
SPWeb newweb = mycol.Add(SiteName.Text, SiteName.Text, SiteName.Text,
Convert.ToUInt32(1033), "STS#0", false, false);