I can't receive SMS when I have low storage From the time the "low storage" warning appears no synchronization is done at all, including receiving SMS. 

Can I do anything about it? Can I "force" my Motorola Milestone Froyo to receive SMS somehow? 

This is really annoying. Especially because the user doesn't informed in detail. I missed a lot of SMS... I hope that gets fixed in Froyo.
SMS, etc. are stored on internal storage.  If it's getting full, then you'll have to uninstall some apps to free up space to receive more messages.  Not much else you can do about this.

My suggestion, however, is that you use an app like SMS Backup+ to backup your old SMS messages to a label in gmail.  Then you can delete old threads which will free up space AND you'll still have a copy of all of your messages.