Okay, new version:
http://dl.getdropbox.com/u/529356/Semi% ... arkIII.cap
A larger blob with more nodes and a more structurally sound skin. You can see the spring force connections here, they're indicated by the green spokes.
You can push itaround with the big red circle, just be gentle with it . It's quite easy to break (and sometimes it even starts off broken, with the 1 node below the 17 node, but you can nudge it back out if you're careful).
I think it needs a more solid skin, so the next step is to make the nodes themselves have no collision, and instead hinge some slender, solid spoke objects around the outside of the blob. That might do the trick. Still, this is the closest I've come to making a decent blob, if anyone else wants to mess around with it, feel free.