I grew interested in Debian lately and downloaded the 7 iso's to try it out, so now I'm reading a lot first to avoid unpleasant surprises.
I read that one could install rpm packages with Debian, even rpm's made for Red Hat (using a program called "alien")?

I'm confused about this, what's the point of the package management system debate if it's possible to install .deb AND .rpm packages with Debian.
That would meand the best of both worlds, wouldn't it?
I even saw rpm repositories for Woody!!

All I should do is # apt-get install rpm and I'm set to go?

It is a little confusing. I'll try to make it simple:

Yes, you can use RPM with Debian to install RPM packages. However, this is not done with alien. Rather, it is actually done with the same RPM program that is used by the Red Hat distro, which is available as a Debian package (you can use apt-get or dselect to install the RPM program).

The purpose of alien is to convert other types of packages to DEB packages. In other words, if you have an RPM and you want it to be managed by Debian's package-management tools, you need to convert it with alien first, because said tools only work with DEB packages.

So you have a choice, once you've downloaded an RPM file: use it as-is (with RPM), or use alien to convert it to a DEB file. I personally prefer the second option, because I think that Debian's method of package management is superior to RPM (specifically, it can handle dependencies and uninstalls much better in many cases), and because that saves you from having to use two package managers and keep track of which software was installed by which. But you may prefer RPM if you are already familiar with it.

If you want to keep a clean .deb system you should try alien and convert the rpm.If you do install rpm's apt doesn't know about them and that can screw up things AFAIK.It is working fine with smaller stuff.I wouldn't try it with kde-app's or openoffice though.You probably won't need this anyway - there is a .deb for about anything.

If you start useing both rpm and debs on the same system it would get messed up pretty fast since they keep track of deps independantly of each other...

I think the rpm package is included for 2 main reasons... 1). To create rpm's if you are a developer 2). The sake of completeness

Debian has thousands of packages. You don't need to install RPMs. Its a waste of time and effort. It took me 20 minutes to convert the SuSE StarOffice 5.2 RPM to a Debian package, and it wasn't worth it. Since StarOffice is junk (IMHO)