Problem with update_usermeta (I think it’s the colons)

I am working on trying to integrate wordpress and bbpress and came across a problem where when people were registering through wordpress, it was not giving them capability to post to the bbpress install. So I created a small plugin that adds the necessary meta key and value for bbpress.
Here is the plugin code:

function insertbbPressRole($insert_id) {
	update_usermeta($insert_id,'bb_capabilities','a:1:{s:6:"member";b:1;}');
}

add_action('user_register', 'insertbbPressRole');

But, the value it is actually inserting into the db is
s:23:"a:1:{s:6:"member";b:1;}";

It makes no sense at all. I think I have tracked it down that the colons are causing the issue. I tried escaping them, and running various functions on the string before inserting it, but to no avail.

Anyone know why this is happening?