pbpasteのman(OSX 10.6.6)を見ると、以下のように書かれているから、

https://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man1/pbpaste.1.html

pbcopy and pbpaste use locale environment variables to determine
the encoding to be used for input and output. For example,
absent other locale settings, setting the environment variable
LANG=en_US.UTF-8 will cause pbcopy and pbpaste to use UTF-8 for
input and output. If an encoding cannot be determined from the locale,
the standard C encoding will be used. Use of UTF-8 is recommended.
Note that by default the Terminal application uses the UTF-8 encoding and automatically sets the appropriate locale environment variable.

OSX 10.7のmanを確認した上で、たとえば

LANG=ja_JP.UTF-8 pbpaste > ~/ja.txt

とやってみるとか