Heres a suggestion (far from ideal) -
1. Drill each end of the crack out with a 3mm drill to stop it spreading.
2. De-tension the rear shock and remove the pivot.
3. Get yourself a good quality seat post, cut the top off so its merely a tube with no right angle, drop it (or force it) down the tube overlapping the crack, then drill it all the way through above and below the crack to bolt it in place.
Its a nasty fix, but it might save you a frame and a lot of very invasive welding, that will ruin all your paint and probably distort the frame.
Sympathy to you bud. Nasty.
EDIT: By the look of it though, the bosses that mount the pivot and top tube may well interfere with the consistency of the seat post, so a tube may not go all the way down it - worth a look though.
Edited by dan0h (9th Dec 2010 12:00am)
Edit Reason: Re-think
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