TiboJ:
Good idea! I've also found a library for it:
http://arduino.cc/hu/Reference/SoftwareSerial
Yeah, that's pretty much the standard library for software serial (or is it the NewSoftwareSerial?) - there are actually several such libraries out there, and I really forget which is the "latest" that is "standard" (maybe somebody here can help me out?). I do know there are a few out there that claim to have this or that improvements over the standard library for certain tasks; you might take a look around.
The point of using a separate servo controller is so you can "offload" the work of controlling the servos to another controller. Note that this is -exactly- what the WildThumper is doing; in its case, the controller is an ATMega168 using the Arduino libraries (specifically the Servo library).
So - if you wanted to - you could easily build your own "custom" WildThumper-clone using the code they provide, your own Arduino, and your own motor controller for the DC motors.
TiboJ:
I'll take a look at this, and search for some tutorials/library's.
As I said above - the WildThumper does everything you need to do, it is just limited in memory; all of the code is published on the site (download it and read thru it - it is very basic and easy to understand, and commented well). The code already handles taking PPM input from the RC receiver, and decoding it for output to the motor driver.
TiboJ:
I think that is a limitation for me, the code is probably going to be big, so the 168 might have to less memory.
If you do as I suggest above (with attention paid to what I note below), you could (in theory) use a 644 or 1284-based Arduino (though you might have to build that yourself) or Mega 1280/2560 as the controller and have room to spare. Or, you could use a pair of 328-based Unos. Or even a single Uno; it is difficult to say what you need or what you can use, without knowing your complete specs of what you plan to do ultimately. For simple RC control, etc - a single Uno would suffice; you could probably even get the OSD stuff I was talking about working as well. But, adding other things like GPS or other peripherals will expand memory requirements, and you might find yourself running low with a 328.
TiboJ:
I'll think about it, and otherwise, there is an alternative:
Sabertooth 2X12 R/C regenerative dual motor driver
Should 2x12A be enough, as each motor requires 5.5A( 3 x 5.5A = 16.5A)?
That is, IIRC, just an h-bridge; it is unclear on the specs of the Thumper whether that 5.5A stall current is for the whole device, or per motor (there are 6 motors on that chassis, btw, not 3). If it is 5.5A per motor, then I would only consider using one h-bridge per set of motors (so you would need 3 of those sabertooth controllers). If it was in total (that is, 5.5A for all motors locked up, so 2.25 amps per each side), then the sabertooth would be extreme overkill, but you could use one. Unfortunately, that site does not give good enough information about the specs on the motors for the Thumper chassis to know what they mean (I would suggest emailing them for clarification and/or reading the manual for the chassis and/or motors).
Ah - reading on the controller page for the WildThumper controller indicates that indeed - it is 5.5A stall current for -EACH- motor; or 33A total for all motors stalled - so you could go with (at minimum) two sabertooth 2x12 controllers; one controlling 4 motors (two per side), the other controlling the remaining two - or use 3 sabertooths, one per pair of motors - or try to find a different suitable h-bridge to handle the motors (about 16.5A per side of 3 motors).
Of course, regardless of the above answer, you would have to build up your own "controller" as outlined before to control the h-bridge(s) involved (based on the WildThumper controller code).
To be honest, you would probably be better off sticking with the WildThumper controller, and using a separate Arduino for the extra peripheral stuff, communicating with it via the 168 on the WildThumper controller via software serial or I2C (or, have the serparate Arduino be the master, controlling the WildThumper using I2C or serial; your call).