Thanks for the guide. Just some hints though, in case someone else stumbles in to it.
What I wanted to do was to upgrade the core, network and maemo5 libraries (so I had for the dependencies also install xml, dbus and something else I forgot). The reason was to test my own app on the device since it actually uses the maemo5 features and the other runtime libraries mismatched of course.
Anyway, what happened was a couple of things:
1. The email app. stopped working (just crashed when trying to start it).
2. Battery drop in just a couple of hours, every time after recharging. There were also temporary "freezeups" for a few secs now and then (guess some process was spinning).
EDIT:
3. Oh another thing, don't be surprised when you go into the upgrade manager and it says that it want to upgrade Maemo5 (to the PR1.1.1 release). It take it that this is just because it discovers that some libraries do not match.

So, be aware, things might get instable. Remember you do not have the PR1.2 installed yet.

But, I have just downgraded again, and it seems my mail is working again. I just have to wait and see that the freezings and battery drops are gone too.