Hi,
I have a 40GB iPOD (it has 4 round buttons above the touch wheel). I
have several iPOD docks around the house, each connected to an
amplifier and speakres. So wherever I want to play the music, I dock
the iPOD.
Now considering an upgrade to the iPOD photo 60GB (the model currently
being sold by Apple Store). The question is: do I have to upgrade all
the docks or will the 60GB photo model fit in nicely?

No i dont belive you have to change/upgrade the docks you currently
own as long as the newer ipod u buy would also be a "spinwheel" type

Yes and no. That is why I decided to ask a formal question here.
If you look at 

http://www.jbl.com/home/products/product_detail.aspx?prod=JBL%20ON%20STAGE&cat=HMM&ser=HMM

or

http://store.apple.com/1-800-MY-APPLE/WebObjects/AppleStore.woa/72503/wo/Yb2fsIF0jTCN3TYeUo61a3zHf5k/2.0.0.15.1.0.6.9.2.11.0.7.4.0.8

there is a remark:
"Attention all iPod Photo/Color Owners: There's now an adapter for JBL
On Stage that's free (except for handling*, freight and applicable
tax, US only) available through www.jbl.com"

This means the original docking bay in the JBL did not fit the 60GB
Photo model. So there possibly is a subtle difference between 3rd gen
40GB and 4th gen 60GB models in terms of docking.