Recently, I was using a regular host kernel 2.4.20 with a 2.4.19-um37 guest.  I was running very heavy rsync (debian mirroring) scripts inside the guest and was having stability problems.  After approx. 10 minutes, The UML would always crash hard and all the tun and bridge devices on the host needed to be reset.
I thought I would upgrade to the latest stuff and try and give you people some decent debugging information.  However, I upgraded the host to 2.4.20 (with the skas3 patch) and the guest UML to 2.4.19-um40 and everything now works perfectly.  The rsync has been running for a solid day so far with no problems.  I am very impressed with UML and I would like to thank those responsible for their hard work (esp. Jeff).  It is such a great piece of work.  I used to use FreeBSD jails quite often to run services, but it was always annoying to setup and maintain multiple jails.  UML is great for jailing services and easy to maintain because you can run full distributions, not strange hacked up FreeBSD images for your jail image.  Also, the performance is excellent!  I also use UML to consolidate many different, complex debian build environments.  It is fantastic!
Thanks all,