
And of course i brought it on myself.
This is for a part of my rear wishbones. I drilled six holes in the end of the tube, and inserted an internally threaded tube, about 50mm long, into
the end. I then plug welded three of the holes, checked for distortion by screwing the balljoint in about ten turns, removed, and plug welded the
remaining nine holes. Checked again that the balljoint screwed in, a little tighter but no problems. Then i start to unscrew the balljoint, and you
guessed it, the tube cooled and the balljoint is now jammed good 'n' proper.
I'm thinking that the tube may have bent very very slightly as some of the welds cooled and contracted, jamming the balljoint. I've tried
applying heat which gave me about 1° of rotation when turning the balljoint with a large adjustable spanner.
Has anyone got any tips besides more heat, clamp it tighter and turn harder?!
All suggestions more than welcome.
Blow torch on the tube and a bit of effort should sort it. Or make a new one now you know what not to do! 
I was thinking of doing the same for the other wishbone, but running a tap through the thread after the tube has cooled. Which is what i should've done in the first place!
clamp the wishbone tight, heat it hot and force the ball joint out. You may damage it, but at least it will be out!
And yes, you often have to re tap after welding, i did and my inserts were hefty compared to the amount of welding i did.
[Edited on 17/1/07 by JoelP]
Righto, i think i'll weld the wisbone together so i've got more to clamp to, then heat and twist. Thanks guys. 
second that Joelp 
Got access to CO2? If so, blast it onto the ball joint thread after heating the tube nice & hot. It should screw out easily then (but I hesitate
to say that it will be "finger tight", for obvious reasons!!)

I always retap after welding wishbones.
I say retap, because I like to tap them to start with in the lathe to get them dead square, and that's not usually possible with a full wishbone,
so I tap the end in the lathe, weld it in, then retap after.
You may find that laying the welded section on an anvil and giving it a few firm hits with a large hammer may help .