This is an idea I bounced off a bit back and I am glad it finally got some traction. A friend of mine asked me about 2 months ago if there was a way to ignore a incoming call and automatically reply with an SMS message. He told me that he had this option on his Treo and really missed it on his 8330.
From the beta description:
NotNow lets you ignore the incoming call and send a friendly SMS message to the caller.
There are times that you can not answer the call such as in a meeting, at the movie, driving or just busy. You can simply press the Ignore button and then select a pre-defined message to send to the caller.
Note: Landline callers may not be able to receive SMS and some carriers charges you for sending SMS to landline numbers. Check with your wireless carrier.
Thanks to Tashanna for sending this one in via PinStack