This seems to do what you want:
\begin{filecontents*}{abcdefg.bib}
@ARTICLE{sievers07,
author = {{Sievers}, J.~L. and {Achermann}, C. and {Bond}, J.~R. and {Bronfman}, L. and
{Bustos}, R. and {Contaldi}, C.~R. and {Dickinson}, C. and {Ferreira}, P.~G. and
{Jones}, M.~E. and {Lewis}, A.~M. and {Mason}, B.~S. and {May}, J. and
{Myers}, S.~T. and {Oyarce}, N. and {Padin}, S. and {Pearson}, T.~J. and
{Pospieszalski}, M. and {Readhead}, A.~C.~S. and {Reeves}, R. and
{Taylor}, A.~C. and {Torres}, S.},
title = "{Implications of the Cosmic Background Imager Polarization Data}",
journal = "Astrophys. J.",
year = "2007",
pages = {976-987},
}
@book{goossens93,
author = "Michel Goossens",
title = "The {LaTeX} Companion",
year = "1993",
publisher = "Addison-Wesley",
address = "Reading, Massachusetts"
}
\end{filecontents*}
\documentclass{scrbook}
\usepackage[T1]{fontenc}
\usepackage{classicthesis}
\usepackage[authoryear,round,colon]{natbib}
\hypersetup{colorlinks=true,citecolor=cyan}% just for the example
\begin{document}
\citep{sievers07,goossens93}
\bibliographystyle{plainnat}
\bibliography{abcdefg}
\end{document}

The problem is that first pages of chapters and similar ones (ToC, etc.) use the plain
style even when you redefine the global style.
In KOMA-script classes this behavior can be changed modifying the meaning of \chapterpagestyle
to be pgnumbottomcenter
, i.e.
\renewcommand{\chapterpagestyle}{pgnumbottomcenter}
MWE
\documentclass[
twoside,
openright,
titlepage, numbers=noenddot, headinclude,
footinclude=true,
cleardoublepage=empty,
BCOR=30mm, paper=letter, fontsize=11pt,
listof=totoc,
]{scrreprt}
\PassOptionsToPackage{
eulerchapternumbers,
listings,
pdfspacing,
subfig,
beramono,
parts}{classicthesis}
\usepackage{lipsum}
\usepackage{scrpage2}
\deftripstyle{pgnumbottomcenter}{}{}{}{}{\pagemark{}}{}
\pagestyle{pgnumbottomcenter}
\renewcommand{\chapterpagestyle}{pgnumbottomcenter}
\begin{document}
\chapter{Test}
\lipsum[1-10]
\end{document}
Output

Best Answer
classicthesis
uses thescrpage2
package to customize the headers and footers; you can add the page to the footer alway to the right with a redefinition of thescrheadings
page style:A detail of the bottom of two consecutive pages: