Impact of SAPUI5 on GuiXT: What is the impact of SAPUI5 on the future of GuiXT? Will we be able to use components of SAPUI5 and GuiXT together?

SAPUI5 is a promising client-side framework still under development according to SAP (as at May 2012). It is not a replacement of GuiXT - it is just an HTML UI framework.
As with any HTML application you can integrate it into SAP GUI using the GuiXT Controls component.