how wide is wide enough? well, having the kit lens as my widest lens, i've thinking about this one and asking a couple of times here and there and got interesting answers...
for the most part as soon as i mention landscape photography someone always says "no! wide angles are not for lanscape shots! you get too much crap in the frame! they are for interiors only and that's the end of it!"
personally i don't buy it because i've seen some interesting wide angle compositions, cool crops, as well as websites of pro's who say that they really like their 20mm ultra-wides (on full-frame body) for landscape shots, albeit specific landscape shots.
so after pondering this one for a while and looking at lens options i went ahead and ordered the pentax 15mm lens. i got it rather than the 10-20mm sigma because:
a) 10mm distortion is about as cool as fish-eye distortion - it's fun but only once in a great while
b) size and weight... i really like having a compact slr with a compact lens (and looking forward to the k-7)
c) although i only recently got into photography i am beginning to appreciate pentax limited primes
so i guess the answer for me was 15mm. your results may vary...