According to http://www.mongodb.org/display/DOCS/Connections the delimiter for connection string options is comma although there is a caveat on the page that states that not all drivers support the syntax yet.
This may be confusing to devs using the .dll only as you need to dig into the source code to find the supported delimiter.

Currently supports:
mongodb://host1,host2,host3/?connect=direct;slaveok=true

Should support as well:
mongodb://host1,host2,host3/?connect=direct,slaveok=true