<p>In the Windows Mobile world you can create a so-called Today plugin that adds content to the phone's main screen -- the one where you see the number of missed calls, unread sms and upcoming events.  Is it possible to do something similar on the BlackBerry?  I'd like to show some important info there, so that they are as visible and as easily reachable as possible.</p>

These things are already displayed on the ribbon depending on the theme.
If you want to create/modify a theme, use the Plazmic CDK.
There are two ways to put individual icons onto the ribbon (similar to icons like mail, calendar, address book, browser, maps, etc):
channel push: easiest if the user is on a BES, now a way to do it for the non-enterprise people on BIS called WebSignals.
write a midp or rimlet j2me application.  Rimlet preferred unless you really need compatibility with other (non-RIM) j2me midp devices.
OK, there is technically a third way, but I really don't think it's serious: MDS Studio.
All the tools are free but you may need a code signing key if you're writing a rimlet using a secure API.
http://na.blackberry.com/eng/support/docs/developers/?userType=21