Each time Apple comes out with a new phone, they update the iOS. So, most likely, iOS 5 will be released to the public on the same day as the iPhone 5. Just as iOS 4 was released with the iPhone 4. And they can ballpark the estimated time of release (Fall 2011) but there is not an official date out there. And the new iOS's are released with the new devices because the new devices are optimized to use the newer iOS versions. For instance, the software on the iPhone 4 makes it capable of running iOS 4 without difficulty, whereas iOS 4 on an iPhone 3G or 3GS would be significantly slower.
Like a typical major Apple release, rumors have been spinning for months, with the latest report putting the iPhone 5 release date on October 21. Going through those rumors out in the market, it seems that the new version of iPhone is just round the corner. Most probably, Apple will be unveiling iPhone 5 in October. However, there is no chance for a further delay to November or December, according to sources.
i live in UK... according to some authentic sources, iPhone 5 release date in UK is November 21, 2011. But according to CNET uk, iPhone 5 will release on 7 September. So the thing is there is no exact release date of iPhone 5 given by apple. China Times Claims iPhone 5 To Release In Second Week Of September. Apple will initially put an order of 400 million iPhone 5 units, also claimed by them. You can get the latest updates on iPhone 5 release date here http://iphone5hacks.org/iphone-5-release-date-september-2011/
If there is one, it will almost definitely be in June/July, as the last 4 iPhone have been released then. I believe a WWDC has already been scheduled by Apple for one of those monthes.
As to if an iPhone 5 will be unveiled - It's looking unlikely, as many trusted sources have said that it will be a software conference (iOS5) rather than a hardware conference (iPhone 5)