I found this BBS post that mentions the new In Season information found here. The poster has kindly provided a translation text, which reads
“Give you’re [sic] sims a break and weed them out of the house! This new ep to the sims 2 make you’re [sic] sims able to go outside, where they can let them self out in the seasons activities, like snowball war,(something i don’t know what is) and games like Marco Polo and much more. They can paddle around in the spring rain, fish in they’re own ponds and grow rare fruits with unexpected outcomes. Whether you build them a winter cottage or a summer hut, will they enjoy every seasons.”
Of course at this point it’s still all conjecture. We won’t know anything for sure until Maxis “officially” announces it, even if more concrete leaks happen prior (like how we were all “in the know” about Pets before they spoke up).
I’ve just found another BBS post which claims this is an insert with the Pets expansion

I guess we’ll find out if this is true in a few days!
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October 14th, 2006 at 3:46 am
GameSection
The story IS true.
We’ve added som english text to our orignal posting.
http://www.gamesection.dk/nyheder.asp?nid=1990
October 20th, 2006 at 10:54 am
lovefrogs12
I got the same paper in my game which I got yesterday, but mine is in English and at the bottom it says - “Available Early 2007″
Also, after it says your sims will enjoy every season - it says - but watch out for unpredictable weather!
October 21st, 2006 at 7:55 am
calinselene
Yip, definitely true, I recon- I got an inlay with my pets expansion too. You should’ve heard me squeal!!!