I am selling my T61 on a Dutch forum, but someone responded to my topic that the warranty is only valid with the original parts that my T61 shipped with.

I had my T60 come in last week, so i swapped some parts from T61 > T60 (HDD, RAM, DVD).

This won't make the warranty invalid because those items are FRU's (Field Replaceable Unit) so they are meant to be replaced (if needed) by the customer right?

Please enlighten me

You can swap any FRU without losing warranty.
Unless your T61 has nVidia, why would you swap a T61 for a (lowlier) T60?
I answered my own Q, the 'new' T60 has Intel gfx but a darned slow single-core T1300.

Ok, thank you

Yes, you answered your own question, T60 has a great Panasonic 9 cell battery that works for 5 hours with T60 (due to Intel and T1300 processor i think). T61 is working max 2.5 / 3 hours due to the battery eating Nvidia Quadro.

I don't need ATi / Nvidia graphics anyway because i don't run any GPU intensive tasks ( i have a Playstation 3 for gaming )

And about the T1300, it is slow indeed, when i copy some big ISO image from my fileserver the T60 is very slow because the CPU is stressed to 100%. I almost have the deal completed for a T2300