With help from Seb, the nagging problem I had when trying to sweaving with Emacs and ESS (http://old.nabble.com/ESS-and-Sweave-td28339734.html) has been solved.
First of all, apply the patch for ess-swv.el, provided by Seb. Second, set the default pdf viewer for ESS by "C-h v ess-pdf-viewer-pref ", then change the value from "nill" to "evince". Now I can directly sweave my Rnw document by pressing "M-n s", and see the pdf output by pressing "M-s P".
It has been a quite pleasant experience interacting with free software developer/maintainer.
By the way, Emacs 23.2 just got out, and it looks great on my Ubuntu box (the font scale problem that comes with the proprietary Nvidia driver is gone).