running gallery from user dir on Mac OS X

Diagnostic info:

Gallery URL (optional but very useful): localhost
Gallery version:1.5.1
Apache version:1.3.33
PHP version (don't just say PHP 4, please):4.4.1
Graphics Toolkit:ImageMagick
Operating system: Mac OS X 10.4.6
Web browser/version (if applicable): Firefox 1.5.0.1

I am trying to use Gallery on Mac OS X, from my user folder. Gallery seems to work fine if you host it in the main webserver folder, e.g.,
http://localhost/gallery/album01
will work. However, if I use
http://localhost/~username/gallery/album01
it won't work. I have mod_rewrite turned on, but the rewrite is failing when ~username is used.

The browser shows a 404 error:
> The requested URL
> /gallery/view_album.php was not
> found on this server.

while the apache error log shows

> File does not exist:
> /Library/WebServer/Documents/gallery/view_album.php

which of course doesn't exist, because the file is actually in
/Users/username/Sites/gallery/view_album.php

I'm not an .htaccess expert, so I'm not sure if this is an Apache configuration problem or an .htaccess bug. Has anyone out there tried this before-- running gallery out of a user folder on Mac OS X?

Thanks,
aaron