SDK - will its release mean that jailbreaking isn't really necessary?

[Raider]

I'm about to order my iPhone, probably officially through O2 with the £35 a month tariff. Once the SDK comes out will it remove a lot of the need to jailbreak?

I assume the main role of jailbreaking is to allow you to run the iphone on non-official O2 contracts. But I assume with the SDK coming out it'll increase the numbers of apps that you can use?