(This question was originally asked before merged mining began)
When exactly did merged mining of Bitcoin and Namecoin chains start?
The switch to merged mining is scheduled for Namecoin block 19200. At the time of this writing, the Namecoin network is on block 18805, so 395 more blocks need to be solved.
The last 29 blocks on the Namecoin block explorer were solved in an average of 108.79 minutes each. Extrapolating from this, 395 blocks should be solved in just under 30 days, so sometime around September 29th.