Programmatically create sub-site AND validate if desired name exists

Check the MSDN page on using the SPWebCollection.Add method (which you are using here): http://msdn.microsoft.com/en-us/library/ms412285.aspx

It has an example code there doing exactly what you are looking to do:

// If a subsite by that name exists, open it.
    string[] webs = parentWeb.Webs.Names;
    if (webs != null && Array.IndexOf(webs, childName) >= 0)
    {
        childWeb = parentWeb.Webs[childName];
    }

    // Otherwise, create the subsite.
    if (childWeb == null)
    {
        string title = "Wiki";
        string desc = "A place to capture knowledge.";
        uint lcid = parentWeb.Language;

        string templateName = "WIKI#0";

        childWeb = parentWeb.Webs.Add(childName, title, desc, lcid, templateName, false, false);
    }

Above is just a snippet of what's there, but I believe this is what you are looking for.

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Brandon Atkinson

Blog: http://brandonatkinson.blogspot.com