Yes! I have considered this myself. It is just exciting having fun things to look forward to, as well as helpful in planning.
A little about our family learning style to this point: We have been unschoolers from the beginning, though I incorporate our faith as well as Charlotte Mason-ish activities, like living books and nature study and other things.
I am going with a year theme for 2012: Warriors, since I have 3 boys who love weapons and heroes.
Warning: If you are anti-weapons, read no further! 
My boys are always making swords and guns out of sticks they find while camping and they really like Nerf foam dart guns and such things.
Each month will have a different focus.
Since there is a really neat Civil War Reenactment close to where we live in the Fall, that month we'll have a trip to see that and I'll take some time to explore various topics around the theme, like Abraham Lincoln, slavery, life in the South, how guns are made, shooting real guns with my brother who is a gun-nut.
I think of it sort of like exploring a topic, not really a unit study, because it's much more fluid than that and relying on where their interests take us as well. It just makes sense to keep in mind what is happening around us and to plan accordingly.
I also like the idea of having a library day--we've done that in the past.
I have so many cool books and games and other things at home, though, that I'd like to put them to good use--so I have considered having a Math Monday, Science Tuesday, History Wednesday, Geography Thursday and Art/Music Friday. That day, in addition to what we normally do, I'd have a few activities marked in a book (or ideas I've gleaned from other families) and offer it up to the kids which ones they would be interested in doing that day and then we'd do the fun activity together. I imagine this would be about 30 minutes, unless they really liked it and wanted to keep going (like the baking soda and vinegar fountains, which was like 2 hours!) I don't think I would keep up the weekly schedule the whole month or year--maybe for 2 weeks on, and then another 2 weeks after a break and see if it it something we all want to continue.
Gonna be fun!
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