Is there a seamless mode for XEN like the solution for VirtualBox?
UPDATE: under e.g.: having Ubuntu 11.04 as dom0. With seamless mode I mean to integrate the guest vm's window[s] to the host's GUI.
As far as I know, Xen does not have a seamless mode.
You didn't mention which guest you want to run seamlessly, but if it's windows, you might be able to get a similar functionality (non-local apps appearing to run locally) by using Windows Terminal Services and RDP.
If you want to run some other guest, then I believe you have fewer options (none that I'm aware of).
Bill B mentioned Terminal Services / RDP as a way to do this with windows guests. You can access just a single window and so easily mix and match windows on your primary desktop by using a network connection and a display protocol instead of the graphical XEN display.
The same effect can also be done even easier for *nix clients by using X-window forwarding.