Count characters, turn them into a cipher
Bash/coreutils, 91 chars
tr a-z `echo {a..z} $1|fold -w1|grep '[a-z]'|sort|uniq -c|sort -rn|awk '{printf $2}'`&lt;&lt;&lt;$1

Save as cipher.sh, chmod +x it and run:
$ ./cipher.sh &quot;Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nunc sed dui at nisi euismod pellentesque ac sed enim. Nullam auctor justo quis diam faucibus, eu fringilla est porttitor. Pellentesque vel pharetra nisl. Vestibulum congue ultrices magna a fringilla. Quisque porttitor, risus suscipit pellentesque tristique, orci lorem accumsan nisi, scelerisque viverra turpis metus sit amet sapien. Fusce facilisis diam turpis, nec lobortis dui blandit nec. Vestibulum ac urna ut lacus commodo sollicitudin nec non velit. Nulla cursus urna sem, at tincidunt sem molestie vel. Nullam fringilla ante eu dolor bibendum, posuere iaculis nunc lacinia. Sed ac pretium est, vel scelerisque nisl. Phasellus lobortis dolor sodales sapien mollis hendrerit. Integer scelerisque tempor tellus, viverra varius neque mattis in. Integer porta vestibulum nisl, et feugiat tortor tincidunt vel. Aenean dignissim eleifend faucibus. Morbi nec neque vel ante pulvinar mollis eu in ipsum.&quot;
Lvgld rqfjd uvmvg frh edlh, svpflshlhjg eurqrfsrpn lmrh. Njps flu ujr eh prfr ljrfdvu qlmmlphlfbjl es flu lprd. Njmmed ejshvg ojfhv bjrf ured tejsrijf, lj tgrpnrmme lfh qvghhrhvg. Plmmlphlfbjl jlm qaeglhge prfm. Vlfhrijmjd svpnjl jmhgrslf denpe e tgrpnrmme. Qjrfbjl qvghhrhvg, grfjf fjfsrqrh qlmmlphlfbjl hgrfhrbjl, vgsr mvgld essjdfep prfr, fslmlgrfbjl jrjlgge hjgqrf dlhjf frh edlh feqrlp. Fjfsl tesrmrfrf ured hjgqrf, pls mvivghrf ujr imepurh pls. Vlfhrijmjd es jgpe jh mesjf svddvuv fvmmrsrhjurp pls pvp jlmrh. Njmme sjgfjf jgpe fld, eh hrpsrujph fld dvmlfhrl jlm. Njmmed tgrpnrmme ephl lj uvmvg irilpujd, qvfjlgl resjmrf pjps mesrpre. Slu es qglhrjd lfh, jlm fslmlgrfbjl prfm. Paeflmmjf mvivghrf uvmvg fvuemlf feqrlp dvmmrf alpuglgrh. Iphlnlg fslmlgrfbjl hldqvg hlmmjf, jrjlgge jegrjf plbjl dehhrf rp. Iphlnlg qvghe jlfhrijmjd prfm, lh tljnreh hvghvg hrpsrujph jlm. Alplep urnprffrd lmlrtlpu tejsrijf. Mvgir pls plbjl jlm ephl qjmjrpeg dvmmrf lj rp rqfjd.
$