Are creature tokens considered permanents? (Magic: The Gathering)?

Yes, a token is a permanent. What happens is that when "boomerang" is cast on it, it does not return to your hand; it instantly disolves (disappears). The same thing would happen if you had to send your token to the graveyard. Once your token is chosen to be sent to your graveyard, than it instantly disolves, disappearing from play, but not appearing into your graveyard. Finally, the same thing can be held true with cards like "Astral Slide" cards that remove your token from the game, and then returning it to play at some later point. Once your token is removed from the game, it is removed, but it also instantly disolves. Since is is disolved, it is not eligable to be returned to play.

Tokens are indeed permanents and have whatever attributes are assigned them by the card that created them. They may be affected by other cards in the same way any other permanent with those attributes could be. If the permanent would go to the graveyard or return to the players hand, it is simply removed from play.

Since your using marit lage as the example then i will also. The answer is yes tokens are permanents. When your opponent using boomerang on the Marit Lage token then it returns to your hand and is gone forever since tokens have no mana cost. It's not that indestructable is it? I hate blue decks.

Yes, tokens are permanents, with attributes assigned to it by the card or effect that created it (a Red Creature, a Colorless Artificat Creature with Flying, etc.)

Techincally, it'll just disappear since it's a token and can't be returned to your hand. And yes, it's considered a permanent. :)

But you could always play Stonewood Invocation and make it untargetable...

Yes. Everything on the battlefield is permanent.

if i remember it correctly tokens are counted as permanents.