is there a (simple) way to move a single TeamProject from one collection to another (in one server) in TFS 2010? Including source code, work items, documents, project site...
No not really simple, you can split a collection and creata a new collection with the team projects you move in it, but you cannot out-of-the-box merge a team project into an existing collection.
What you can do is to use the TFS integration platform http://visualstudiogallery.msdn.microsoft.com/eb77e739-c98c-4e36-9ead-fa115b27fefe or http://tfsintegration.codeplex.com to migrate your team project over to the new collection. This is however a DataMigration that doesnt keep or even move all data, but it will move Source Code & Workitems. Moving a team project site (if needed) , including documents can be done by using the STSADM Export and Import commands, please refer to http://technet.microsoft.com/en-us/library/cc288191%28v=office.12%29.aspx for more info about moving SharePoint sites