Hello,
I'm searching about how to have different panels in the different activities,
e.g : a panel in the first activity and three in the second one.

Is there a way to do that in kde 4.5 ?

No, that's not possible. I'm not sure if it's considered for future KDE-releases but right now it cannot be done.

Thanks for your answer, I didn't find this function in the plans nor in the wishlist so I'll post an enhancement "bug" at bugs.kde.org

I would love that possibility (searched before this post for this functionality, .....)
=> but there must stay an option to use the same panel on all activities....

It would be the best improvement for the 'different' activities thing
: they all would be completely different activities

I posted the wish : Bug #249141

Thanks

I do recall seeing posts (brainstorm?) about a related feature request, though airc the request was to have plasmoids shared between multiple activities; so kind of the other way around, but airc it was mentioned that if that request went through that they would use that mechanism for panels (which would in turn allow per-activity panels).

As it currently stands, the panel is basically its own activity (formally called 'containment') that is independent of the other activities.