Connection string syntax is defined here:
http://www.mongodb.org/display/DOCS/Connections

The shell does not appear to support this syntax at all.

Doing something like the following throws an assertion or simply crashes the shell:

var other_server = new Mongo('mongodb://localhost:6900,localhost:6901 '); // fails
var other_server = new Mongo('mongodb://localhost:6900 '); // also fails
var other_server = connect('mongodb://localhost:6900 '); // fails