We've known that Telstra would be losing the 2100Mhz part of its 3G network for some time; it was a legacy of the early 3G tie-up between Telstra and 3 due to be switched over in August. Telstra's started notifying customers early, so if you're still on an 2100Mhz 3G phone, you can expect a letter just like this to drop through your letterbox any day now.
Telstra will still operate its 850Mhz "Next-G" network into the future, but if you're on an older device, your time is limited, as per the letter that Telstra customers are getting. Yes, that's right; a physical letter. Thanks to Fester for sending this in; I was somewhat surprised Telstra didn't notify customers via email or SMS, but there you go.
Update: Fester tells me that when he called through, he was told that as the service was ending, he'd be let out of his contract. That's not confirmed by Telstra at this stage, but could be worth checking if you did happen to be on a 2100Mhz-only contract phone product.