The latest REST api supports version negotiation:

The client may indicate what service (protocol) version he'd like to use and the service would then use this version to talk to the client:

Mail by Lili Nader on the 4.21.2012

The rest api will return the latest version unless a version is explicitly requested by specifying it in HTTP Accept header in this format "Accept: application/xml; version=1.0"

For example:
curl -i -k https://localhost/broker/rest/api -H "Accept: application/xml; version=1.0"

The api also returns a list of supported versions (right now the only supported version is 1.0):

<?xml version="1.0" encoding="UTF-8"?>
<response>
<version>2.0</version>
<supported-api-versions>
<supported-api-version>1.0</supported-api-version>
</supported-api-versions>
</response>

If the version requested in not supported then it will return a HTTP Status 406 and an error message as shown below.

For example:
curl -i -k https://localhost/broker/rest/api -H "Accept: application/xml; version=2.2"
HTTP/1.1 406 Not Acceptable
<?xml version="1.0" encoding="UTF-8"?>
<response>
<version>1.0</version>
<messages>
<message>
<field nil="true"></field>
<text>Requested API version 2.2 is not supported. Supported versions are 1.0</text>
<exit-code/>
<severity>error</severity>
</message>
</messages>
<status>not_acceptable</status>
<supported-api-versions>
<supported-api-version>1.0</supported-api-version>
</supported-api-versions>
</response>