Apple releases all over the map. The last two iPhone updates have been Tuesday afternoons, and they weren't hurry-up rushed-out fixes, either.
The Friday evening release tradition is a myth. Really, if you look back, there aren't many Friday evening drops from any tech company.
It may be Friday, but that could be just because they pegged this one to Friday. It is still unusual however for Jobs and Apple to say "later" in the month and mean last business day, as when they mean the last business day of the month, they always say "by the end of the month" or "by the end of September [or whatever month] -- that's always then last business day. But they keep saying "later", which makes me think before Friday is the scheduled date, or as soon as absolutely possible when they finish everything and test it, Friday being the latest day they'll release. I know that all sounds silly, but large high-profile company's have a public terminology and they typically use it to a standard, often even a documented standard.
Also the timing of Monday's press release about unlocked phones. PR people, of which I was one and know many still doing PR, will tell you if you need some event to sink in with people you announce it about 36 - 48 hours in advance, mostly to give the press time to run it and people time to pick up on it -- and you always hope war doesn't break out or something like that, as you'll get no press -- but if you send out your release four or five days in advance, the daily and hourly press will cover it within 24 hours, keep mentioning it, if you're lucky, through hour 36 to 48, then drop it. And by the day of the event two or three days later, everyone will have totally forgot they were invited, some product goes on sale, or they were warned about something. So if it is Friday, Monday's release timing was bad PR timing.
Apple likes to release updates on Friday evenings, historically.
Plus, I'm giving them the most time to get things right, since Apple obviously has no iPhone quality assurance at all after going through 4 bad iPhones at the Apple Store yesterday.