Further, the fact that this was caught so quickly is in some sense a vindication of Telegram's model - even in its infancy when it had orders of magnitude fewer users, the fact that the client was open source allowed someone to quickly spot a vulnerability.
The verdict? IMO Telegram secret chats are probably secure (90% certain), but if I were plotting a murder or something, I wouldn't do it over a smartphone app anyway. There's just too many leaky, complex layers in the stack, some of which aren't even open, and quite dubiously so. If security is a life-or-death situation for you, you'd be a fool to use any smartphone app.