You can’t run Silex apps directly on Dreamhost yet. After a bit of digging around I came up with this.
You need to create a file ~/.php/5.3/phprc in your home directory, not under the website directory and put these lines in there.
detect_unicode = Off
phar.readonly = Off
phar.require_hash = Off
suhosin.executor.include.whitelist = phar
Its not immediate, but in about 15 mins it should work.
Vote up my answer if it works for you.