Let's see what Microsoft use on the .Net framework (courtesy of the Shared Source):

System.XML: Same Line
System.Net: Same Line
System.Text.RegularExpressions: Same Line
Microsoft.JScript: Mix - Namespaces on newline, the rest on same line
System.Collections: Same Line
System.Diagnostics: New Line
System.Globalization: Same Line
System.IO: Mix - ad-hoc mixture
System.Reflection: New Line
System.Text: Same Line
System.Runtime.Remoting: Mix - ad-hoc mixture
System.Threading: Mix - ad hoc mixture

When I wrote C, I used to use New Line and during .Net Beta1, I also used New Line, but I switched to same line around Beta2 and have never looked back.