I'm pretty sure some of you have found this out themselves already, but there is a way to exploit Kamino pretty heavily as a rebel player.
Yesterday I tried (in GC) to attack only with 1 tactic:
-build 1+ infiltrator training facilites on Kamino
-continually build infiltrators and group them in raid fleets of 2 units
-raid enemy panets and press "Auto-resolve" (the last being optional)

Using this tactic, I was able to strip the 2 planets I attacked of everything except heroes in a short amount of time and for the moderate cost of about 10 credits (and these weren't lightly defended, one time a raid fleet of 2 infiltrators roughly took out a heavy factory, 2 squadrons of AT-ST, 4 sqadrons of Stormies and 2 scout trooper units).
The problem is that I ATM don't see an easy way to prevent a rebel player from spamming enemy frontline planets continually with infantry raids that cost him practically nothing...

Do you have any ideas?

I'd done that with infiltrators in the original EaW...those guys are pretty awesome if managed properly in a land battle. I think that the strategy has two weaknesses:

(1) It's most effective against an AI player that doesn't adapt to your tactics. Try it against a human, especially if the human Imperial player has TIE Maulers on the ground.

(2) A Rebel player could do what you did for 1000 credits, which isn't too hard to come by. But doing it as cheaply as you have hinges on one planet, and there's the weakness: what if the Empire drives towards Kamino and takes it from you?

All in all, I think that what you describe is fair enough for the Rebels to attempt (and it's perfectly in keeping with canon Rebel strategies) and it's not "overpowered" because a smart Imperial player could counter it without too much extra effort.

I have found that this is usually not necessary, at least for many planets. As a rebel, I usually use any planet to make pairs of troops (usually a plex and a standard soldier). Kamino is good in this patch since they each cost 1 credit...

Send them onto planets either early on or later onto planets that the imperials haven't had time/cash to invest with ground bases. With proper use and tactics, I've been able to take down planets with anything short of heavy factories with just these two squads.

The secret is to immediately search out a build pad (hopefully you know where they are). Build bacta stations then chose whether to garrison the bunkers or not, placing additional build pads into anti-vehicle/anti-infantry turrets for "safe zones".

Now, using caution it's not overly difficult to use the troopers to scout out and engage enemies, drawing them back towards your turrets if you get into trouble, disengaging them to heal at the bacta stations when necessary. It's even easier if there's an indigent population supporting you on that planet.

As an example, I currently am trying to take over Dantooine as a rebel. I've landed my plex and soldier squads and have them garrisoned in several bunkers. I've managed to build anti-infantry and anti-vehicle turrets along the equator of the map such that the imperials have difficulty even getting into the southern half.

I'm currently using the human settlers that are spawned to run raids into the imperial's stronghold, knowing that I'll lose them over and over. In this more extreme example, I'm over half way done with taking a plenet with two barracks and one or more factories (I know there's at least one light due to the ton of ATSTs I've run into). I'm doing this with just the two squads and have lost none as yet.

In this case, without the indiginous population I would probably have to resort to sending in infiltrators, but I'd usually discourage using auto-resolve. Any competant player should be able to inflict more damage against the computer than the auto-resolve. You might even surprise yourself and take the planet without losses.

I managed this so far on five imperial planets all with at least two military buildings in place. All without troop losses.

Nice, Well obviously this works tons better with the help of a character. Obviously couple a unit of troopers with a MPTL would work well, but then again MPTL works best against most stuff