If you don't want the framed text to break accros pages you can use the adjustbox package for this:
\documentclass{article}
\usepackage{adjustbox}
\begin{document}
\adjustbox{minipage=\linewidth-2\fboxsep-2\fboxrule,fbox}{Four score and seven years ago our fathers brought forth on this continent, a new nation, conceived in Liberty, and dedicated to the proposition that all men are created equal.}
\end{document}
This places the content in a minipage environment with the current line width minus the space needed for the frame. If you want the text to be as wide as the normal text and the frame should go into the margin use \adjustbox{minipage=\linewidth,fbox,center}{..} instead.
You can of course use a suitable minipage inside a \fbox manually if you don't like to load another package, but adjustbox makes live a lot easier. Check out the manual for more useful keys.
\..boxmacros are designed for smaller pieces of text and place the content into so called restricted horizontal mode where no line breaks or justification is applied. If you want a frame around a larger running text you are using the wrong macro. – Martin Scharrer♦ Jan 27 '12 at 8:23