Ok I have absolutely NO IDEA how other A8 manufacturers do it but I can assure you it is either soldered to the CPU or the motherboard on a phone

I cannot understand too well what you are saying on the last paragraph, I believe you are asking why we cannot simply buy the same A8 as on the N900 but with more RAM and replace it. I will explain why this is not possible.
First lets think of replacing a computer processor. I do not know if you have ever done it but the part that connects to the motherboard looks like this:

That is easy to replace as one simply removes the old one and places the new ones over the pins and we are good to go.
Now let us go to the phone world. We do not have a PC case in which to fit components, plus users do not upgrade phones. Adding a solution like on a PC would make the phone very fat for a niche market that really does not exist. So in order to save space the CPU is soldered into the motherboard. Soldered means it cannot be unclicked. In theory one can de-solder that CPU (which is pretty much impossible without damaging the motherboard or other components) and solder anotherone on but, with such small pins soldering to the phone is again almost impossible to do and you will not find anyone that can do this with the solderer we all have at home.

If you mean desoldering the RAM from the CPU and soldering in a new, bigger module this is also impossible due to the amount of connections on such a small area that would inherently damage the CPU too. They are a single module and cannot be detatched/reattatched.

Even if you managed any of these solutions it is probable (again I do not have a very big knowledge on the N900's inner working) that the motherboard is not compatible with a different CPU or with more RAM, rendering your device useless.

The bandwidth on the RAM is very big so we cannot produce another interface ON A PHONE that is fast enough to make a swap space (using memory as RAM) of any use as the one integrated in the phone will probably be faster.

On a phone one cannot add more RAM, otherwise you would have seen it done by someone on this community, cmon every hardware hack exists for this phone

Cheers and I hope this helps you

P.S. Please clear up what you meant on the last paragragh, specially the last sentence, as I really do not understand it.

EDIT: It seems skykooler won me to it :P