Also keep in mind that iOS 4 had a lifetime 3-4 months longer than the previous 3 iOSes. Taking this into account, the average update frequency is about 51 days, which isn't that much different than iOS 1 and 2 at about 56 days.
Even disregarding that, I don't think you can jump to the conclusion that the intention is to thwart jailbreaking more than they have in the past. Only one of those nine updates was specifically to address a jailbreak method, which also happened to be a general security hole; Imagine if a more malevolent comex developed something like jailbreakme.com. All of the other updates were accompanied by at least bug fixes.
Like Europa mentioned, I think their hardware updating speaks more to their anti-jailbreak strategy than their software update frequency does.