Before you go “Save As” -> Lib Feat Part (*.sldlfp), you need to PRE-SELECT the sketch. Otherwise when you create the Lib Feat Part, it will be empty (initial sketch gone!).
I came across this note on SolidJot tips for Weldments which was in SW2006, and Yes, this pre-select requirement is still needed in SW2009. I remember this because it happened me a few times, and at the time I thought it was an intermittent issue.
Also just to point out that it is really easy to create weldment profiles, allowing you to easily create structures, frames, patio deckings, railings, chassis’s, etc.. Some people think that if a profile is not in the Ansi Inch or Iso, it can’t be done. Oh yes it can. And SW2009 makes weldments much easier by auto-trimming members, allowing to change the trim-order etc. A nice post including video can be found on rickyjordan.com