You can get a list of available encodings using the command:
pdftotext -listenc
and pick the right one using the -enc argument. Mine here seems to do UTF-8 by default. i.e. your "UTF-8" is superflous
pdftotext -enc UTF-8 your.pdf
You may want to check your locale (LC_ALL, LANG, ...).
EDIT:
I downloaded the following PDF:
http://www.i18nguy.com/unicode/unicodeexample.pdf
and converted it on a Windows 7 PC (german) and XPDF 3.02PL5 using the command:
pdftotext.exe -enc UTF-8 unicodeexample.pdf
The text file is definitely UTF-8 encoded, as all characters are displayed correctly. What are you using the text file for? If you're displaying it through a web application, your content encoding might simply be wrong, while the text file has been converted as you wanted it to.
Double-check using either a browser (force the encoding in Firefox to ISO-8859-1 and UTF-8) or using a hex editor.