This is obviously waiting on the UK press event. It's really a matter of whether they decide to release it this evening-ish stateside time zones, after UK business is thoroughly closed for the day, or they decide to release it stateside several hours after the UK event when Apple headquarters opens for business Tuesday morning.
You're right, though, about "later in the month". Apple has detectable patterns how they loosely announce release dates. Coming this spring means June, maybe a few days before official start of summer. Likewise, available this fall, the last couple of days before winter officially begins.
If Jobs had meant end of September, by the pattern he would have said "coming to iPhone before October" or "before next month", which would have meant the 28th, last business day before October. Later in the month, according to Apple speak, usually means about 10 days to 2 weeks from the day its mentioned.
In my opinion, there's no way they'll roll out the new features in the UK without doing the stateside update within several hours before or after the UK announcement. And there's no way they'll announce in the UK without the WiFi Store on there. Remember, they're losing revenue every minute a million iPhones can't buy on the WiFi Store -- in fact, I think if they were having problems with other parts of the update, they would just strip them out and only do the WiFi Store to get it deployed. So this thing is done, they just have set a time they're going to release it. A time we don't know. 7 or 8 EDT today wouldn't surprise me (Appleinsider mentioned a source said "as early as Monday", which would of course mean the source said some time Monday and Appleinsider is just hedging). Mid-evening EDT is late enough not to waffle the UK announcement, but early enough avid iPhone owners will already have it when they wake up and the details of the UK announcement are clear -- or they can immediately grab it before heading to work. Also late enough that if there are problems there are only very few active hours left before support and engineering open again Tuesday morning.
Well, it's been over an hour and it's still there.
Well, today counts as later in the month.
C'mon Apple, that space between the Settings and iPod icons is starting to look empty. And I have $10 in iTunes credit just waiting to get spent.