This is an activated ability, right? I.e. inside the brackets it should say ": If this permanent etc", right?

Since Alex is talking minute detail, may I suggest:

(: If this permanent is a creature, it becomes a Mountain. If a Mountain, it becomes a creature. Either way it gains this ability, which can only be activated when you can cast a sorcery.)

If we're going to go there, it should be ": If this permanent is a creature it becomes a Mountain land. If this permanent is a land, it becomes a creature. Terraform only as a sorcery."

You don't ever need it to gain the ability, since its abilities aren't affected by its type. In the mechanics name, I suggest getting rid of the hyphen, since this isn't an ability word. I think its name should be phrased like Mountaincycling or Mountainwalk.

Mountainform ? That makes sense.

I like the ability. It's a shame that it's easier to make this a creature first... but all the extra words required to make it a land first would get very annoying...

I think DrugsForRobots may have had a good reason for saying "and gains this ability". When something becomes a basic land, it loses all other text: hence why Dream Thrush and Spreading Seas work as hosers for evil nonbasic lands.

I like this. Maybe rename it "Sleepy Giant"? Or have it " or when this ETBs: ..." ? So that it immediately goes to sleep?

It does end up being a lot of words for a very natural concept though, which is a shame.

It's ALMOST a justification for a flip card.

I think this works... I'm not sure, does it look right?

Also, I believe another user has had the idea of 'land morph' before, credit to that person who inspired the idea. Initially I had wanted something along the same lines, but a little different. Now I believe it more closely resembles their version of the concept. Despite searching, I couldn't locate the card/post/set that I had drawn this from. :(

Yeah, I've seen it suggested a few times. The best was definitely a custom set called Verdia, by a designer called Fallingman. The page about it is still up, but the full spoiler appears to have disappeared from the net. I have a local copy - I should import it into Multiverse. (Edit: I did.)

Apparently Wizards discussed it as a possible mechanic for Zendikar. They claim there were rules problems, but I personally don't think there's any rules problem there that couldn't be easily overcome.

Well, there's the whole "Is that a morph land or a morph creature" thing.

But they did double faced cards, so they have NO room to talk.

I'd like to point out that this is now considerably better than Stalking Stones, which still maintains a Gatherer rating of 3.456. I'm rather aware that the stones are still a land when they creaturize... but the versatility of this card is very, very strong compared to the Stones, which most control decks are perfectly happy to play.

On a side note, this ability really should be sorcery speed. Otherwise, when your opponent attacks, you can unform for 2, Dodge a Doom Blade for 2 more, reform for 2, block, then unform to prevent combat damage for 2 more. I'm pretty sure you didn't envision this card to house the ability ": Target attacking creature becomes blocked."