I'm using Inventor 2010. When I need a wrap-pattern of a pipe so the shop guys can use it to trace on the pipe ends, I derive the original part into a new sheetmetal part, slice it long-wise like Sam said, then delete all outside faces and keep only the center face. Then thicken it, then select the inside face and flatten it with the correct sheetmetal settings (usually k=.50 for rolled shapes).
The reason I slice it that way without using Rip is b/c sometimes rip doesn't work. The reason I Derive the part is b/c w/the FrameGenerator the copes are not 'true'. So I have to do it manually. Doing it manually also lets me put the rip exactly where I want it.
Also, another reason I do it using a Derived part is b/c there may be multiple parts in the model, and if I split the parent part instead of Deriving it into a new part, then that part won't Roll-Up in the BOM, so it messes up my BOM. Using a Derived part bypasses that problem. That usually happens when using FrameGenerator. However, if you use that one part through out your model, then you can just do the same thing to that part and not waste time Deriving it, b/c it won't mess up the BOM. But then you'd have to cope it 'true' or else it won't unroll. Lotta options when doing this.
Also remember you need to used the child view (the new sheetmetal part) for the flatpattern on the drawing. Then if the parent part changes, make sure you manually check the child part to be sure it updated properly so the drawing view is correct.
... Chris
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