Series 60 is explicitly based on Symbian OS 6.1, as specified in the Series_60_basics.pdf document that can be downloaded from www.forum.nokia.com. I suggest you read this document, as your question really doesn't make a lot of sense.
Nokia 6600 is Series 60 v2.0 phone and indeed the symbian OS version is 7.0. All I have heard about the new SDK is that it is going to be available soon, dunno really what the soon means, but propably it's going to be available before the phone comes to the market, in case you can get the demo phone from nokia, then you should be able to get the early-beta versions of the SDK as well, othervise use 1.2 version of the series 60 SDK, and wait.
Basically the software made with 1.X SDK, is not supposed to work perfectly with 6600, but I'm quite sure when ever they get the phone SW ready, well done apps have good change to work on it. anyway I suppose the porting from app working in 3650/7650 to work on 6600 will not be really ahrd stuff anyway.
My appologies for posting an incorrect response.
But from the "Series 60 Platform Basics" document, and I quote:
"Series 60 devices are based on Symbian OS 6.1, an open, robust, multitasking operating system designed for data-enabled mobile phones."
So it that only refering to Series 60 Platform 1? Is that the same as Series 60 v1.x? And therefore Series 60 v2.x would be based on Symbian OS v7.x?
And therefore Series 60 simply refers to "small power constrained devices" with "a large color screen (176 x 208 pixels) and the various inputs (two soft keys, five-way navigator and several dedicated keys)."
Series 60 is an evolving platform and its versions 1.x are based on Symbian 6.1. The following phones are based on these platform versions:
Nokia 7650, Nokia 3650, N-Gage, SX1. Two last ones are not in the shops yet.
New version of the platform 2.0 is based on Series 60 v7.0s. It is be used in Nokia 6600, which will be out in the shops later this year.