I was thinking, "why not just buy a standing desk?" But then I saw the prices :-). Well, if you have the right tools, you can build quite a great adjustable desk, but you'll need more than IKEA. Off the top of my head, here's some things that you could use to build a manual or electric adjustable standing/sitting desk:

Metal tube - http://www.metalsdepot.com/products/alum2.phtml?page=tube... ;

Threaded rods - http://www.ebay.com/itm/3-4-6-x-3-ft-ACME-Threaded-Rod-Gener...

Gas springs (several of them would hold the desk pretty firmly, and you can adjust by pushing it down, with some locking mechanism in place) - http://www.ebay.com/itm/Two-USA-Made-Liftgate-Lift-Supports-...

Reduction motors - http://www.ebay.com/sch/i.html?_trksid=p5197.m570.l1313&...

OR steppers (if you go all out) - http://www.ebay.com/sch/i.html?_nkw=stepper+motor&_sacat...

I guess the work involved is not for everyone, but it could be a fun project :-)...