Clem - there is certainly some distortion, as with any very wide lens, and as has been pointed out, this is most pronounced towards the edges. But this can be used as part of the composition - as I suggested in my earlier post, you should check out the Landscape and Seascape group on Flickr, at:
Flickr: Landscape and Seascape (Com. 2, Invite 1)
An amazing amount of talent out there!
You can post-process some of the distortion out of the pictures, but the most common software to do it - DxO Optics - doesn't cover the Sigma 10-20 on the Pentax yet. But PT Lens does - I bought a copy yesterday for a measly $15 online, and it works quite well. Of course, there is some cropping involved in the auto-corrections, and this does remove some of the advantage of having the wide angle in the first place, but everything is a compromise. More correction = more cropping, and vice versa.
The distortion of perspective you ask about is a characteristic of all wide angle lenses, and the best results rely on good composition - again, see some of the examples in that Flickr group.
And for some of the most exquisite pictures, take a look at Helga Kvam's work - here is a set of peoples' favourites from her Flickr site:
Your Favourites - a photoset on Flickr
If you like them, go to her main photostream and prepare to be amazed. And she uses a 10-20 for a lot of that stuff....
If it's good enough for Helga Kvam, then it's good enough for me