Having just replaced my PSU with the official replacement PSU from GlobalScale Technologies, I thought I would add some details about the procedure to help others.
1. I ordered the replacement from the GlobalScale website and had it in a week, exactly as they promised. You can order it here: http://www.globalscaletechnologies.com/p-36-sheevaplug-power-supply.aspx
2. I opened the case of my SheevaPlug following the directions in the document available from GlobalScale:http://www.linuxshoppen.dk/pdf/produkter/sheevaplug_how_to_replace_psu.pdf
3. I was able to proceed through steps 1 to 5 with no problems. However, on step 6, I discovered that one of the plastic tabs under the screws holding the AC inlet in place was already broken and in the process of prying the AC inlet out, I broke the other one off, as the instructions warn you not to do. But there was no reason for the warning because the replacement PSU has a new AC inlet so it doesn't matter whether you break the old one.
4. I had a fair amount of trouble getting the old PSU out until I noticed that the plastic "locking pieces" described in step 7a are actually locking onto the thin metals sides that contain the old PSU. Once I dug my flathead screwdriver into exactly the right place and pulled up on the cables coming out of the PSU, I was able to release each corner and finally the whole thing came out.
5. In step 9, the directions say to "prepare a new AC-DC module", without telling you how you are supposed to prepare it. It turns out that you need to remove the black plastic insulation cover from the bottom of the new PSU, which is how it is shipped -- at least mine was, and insert the PSU in place of the old one. Instead, I put the new PSU in place with the black plastic cover still on the bottom. Nothing fit very well and I'm lucky I didn't damage anything putting it into place. After a bunch of fiddling and looking carefully at the instructions, I realized that they meant you to remove the plastic cover first before installing the new PSU. Once I did that, everything fit much better and I now had the insulating cover ready to install at step 17.
6. Steps 10 - 15 are the most difficult part of the whole procedure, but mainly because of the instructions. They show the AC inlet being angled down into the piece it has to fit into (step 11) but when I did it this way, I was completely unable to get it to snap into place, even though I took great care to line up the pieces just like the instructions say. After many tries, I was almost ready to give up on the whole thing when I figured out the correct way to install it. You need to place the AC inlet so the screw tabs are lined up with the screw holes and resting on them and then force the other end of the inlet into place, making sure you have proper alignment as shown in step 12. If you have everything aligned, the end of the inlet away from the PSU will snap into place with a satisfying "crack".
7. The rest of the steps are straightforward except for one small detail. In step 20, the small rubber "stands" have a groove in them that needs to be aligned with a ridge in the hole they go into. You can see this in the step 20 photo but no mention is made of it. Be sure to push the stands all the way in or your SheevaPlug will rock back and forth when placed on a flat surface.
That's it. It was a lot trickier than I thought it would be but definitely doable by anyone that's good with a screwdriver. Hopefully, these details will prevent others from making the same mistakes I did.
By the way, once I buttoned up the case and plugged in the SheevaPlug, it powered right up and worked just like new.