Microsoft suggests you to not to use jQuery to interact with Microsoft Dynamics CRM 2011 forms. Use the Xrm.Page object model methods to access form elements.
The only supported use of jQuery in the Microsoft Dynamics CRM 2011 and Microsoft Dynamics CRM Online web application is to use the jQuery.ajax method to retrieve data from the REST endpoint. Using jQuery to modify Microsoft Dynamics CRM 2011 application pages or forms is not supported. You may use jQuery within your own HTML web resource pages.