Don't know why (or with what Update Rollup) event.Mode was removed, but in UR-15, event.Mode does not work as documented here:

http://msdn.microsoft.com/en-us/library/cc150868.aspx

It's alright though, in CRM 2011 they have a new construct documented here you can change your code to utilize.

http://msdn.microsoft.com/en-us/library/gg509060.aspx

Just  a quick tip for your Tuesday in case you install UR-15 and bump into this!

UPDATE:  It helps to understand how to pass the context as the first parameter as shown in the blogpost below, if you are having trouble after reading the documentation below, check this out:
http://rajeevpentyala.wordpress.com/2011/12/13/jscript-validation-on-activationdeactivation-of-record-in-crm-2011/

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