Ok, I took a strange interest into your problem, and this is what I found out:

First of All: Do a search on these forums, it will help find the answer.

Ok, back to what I found:

There are two things you can do: Native ARM Citrix client, and Java Citrix client

Native ARM client:
This only works on OS2005, so it wont directly run on the N900. However, because it is a Debian app, you might be able to run it in EasyDebian or something like that on the tablet. May work, may not, but I would say this is a good bet...

Its a tar.gz file, so I am guessing there are a few binary programs and a sh script that puts them in the right place.

Java Client

There is also a java client, but Maemo does not have java built in. Once again, you can use EasyDebian, or you can try and install something like jalimo, which is a open-source java runtime. I don't know how this works, but people on this forum can help you with that.

Note:

All of these solutions are somewhat messy and not one-click installs. If Citrix comes out with a one-click solution for Maemo, that is the best. But the above should work.