Do you know if it is running SNMP? If it is can you get access to the lan to sniff traffic? If you said yes to both of the above and it's running IOS and not Catos it is very possible you could Man in the middle attack the lan with Cain & Able. Once your APR Poisoning you just capture the SNMP string and use Cains CCDU (Cisco Config Download Utility) to SNMP pull the running config. Once you have the config you can get the plain-text or Cisco type 7 encrypted password (type 7 password can also be decrypted with Cain). This is just one more reason to make Chicagocon as I am going to be presenting all kinds of fun things you can do with Cain & Able!
Answer to your question yes it is possible to force a Cisco Router or switch to give you it's config with out turning it off or stopping traffic. It all depends on how it was setup and if it is running SNMP.
CISSP, CCSE, CCNA, CCAI, Network+, Security+, JNCIA, & MCP