It will most likely be on a Friday, just as the last three, so the 7th (or 8th, isn't the new phone always introduced on Tuesday?) is out.
I've been saying it may be on the 11th, with 4.0 being released the following Wednesday (16th), to save the strain on Apple's activation servers. If not the 11th, then the 18th with the 4.0 release on the 23rd.
At least, I hope they release 4.0 after the new iPhone to help alleviate the strain on the activation servers.
(Remember, once a new OS is installed, it must be activated again on Apple's servers, the same ones used to activate new phones)