As well as seeing the iPad 3 announced later on today (6PM UK time), Apple will likely announced iOS 5.1 and will also likely make it available for immediate download after the event.
iOS 5.1 has been in beta 3 for a good number of weeks now, perhaps making it the longest beta version of iOS ever. The version that developers have is expected to be different to what will be launched tonight in that a number of new features not available in the Beta might crop up. Those features could include the new camera access (slide to capture) function seen a few weeks back as well as bring Siri to the iPad 3 when it launches.
From what we understand, the 5.1 GM (Gold Master) has already been released internally at Apple as well as to various carriers and partners although developers have been stuck at 5.1 Beta 3 for a few months now.
As well as the swipe up to access the camera feature, Siri is also expected to support Japanese. We also expect the UK and other counties out of the US to finally get business search and all of the other features missing from Siri. We also expect Siri to be expanded a little more, perhaps to allow 3rd party access to the API although that part is merely wishful thinking at the moment.
THe build number of the 5.1 GM is 9B176 although that will change when the new version gets signed for public release. Although it could be tonight, if not it should be released within the next few days just ahead of the launch of the iPad 3.