Please allow the option to allow the Clam Database to be updated when a scan is started. This would almost negate the need to update at other times (providing a reliable internet connection) and would allow for the scan to be the most accurate available.
The only disadvantages that I can think of are that scans will take longer (it will include the time taken updating), and that if this is selected as the only update option, and if the connection to the update server goes down (user or server side) then the database may be very out of date.
The way to fix the first problem is simply to make this optional (and possibly off by default for faster scanning times), and for the second problem, simply allow other update options as well.
Please say if there are any other suggestions or problems, and if you'd be prepared to include this.