As mentioned in my previous post I contacted Sony asking about a simple memory upgrade voiding the warranty. Here’s what I said to them.
I purchased a Vaio VGN-NW23GE notebook from the Somerset Mall branch of Dion Wired today and asked that an additional 2GB of RAM be added to the standard 2GB. I was told that this could not be done for Sony Vaio machines without invalidating the warranty, and was given a name and number of someone at Rectron that would be able to arrange the upgrade for me. I thought this a little strange and on getting home and starting the machine I noted that the hardware manual contains a section explaining how to add and remove memory. As this procedure is explained in the user manual it seems unlikely that doing so would void the warranty. You may wish to look into the reason for your retailers having this seemingly incorrect information. I would be most interested to receive feedback on what you find out.
To their credit they replied with the following.
Dear Steve
Thank you for contacting SONY South Africa. I am happy to assist you with the memory upgrade query. When upgrading any of VAIO electrical components it is highly advisable to send in the machine to the authorized service center, this ensures that the upgrade performed is according to the service guide precaution and standards.
The procedures described in the Adding and Removing of memory modules under the hardware user guide, assume familiarity with the general terminology associated with computers and with the safety practice and regulatory compliance requirements for using and modifying of electronic equipment. SONY does not
Kind Regards
Your Sony customer care Team
Unfortunately the response has been truncated, probably by a web based contact system (I had submitted my question via a contact form on their web site). It seems that the act of upgrading the memory alone will not void the warranty but that Sony may refuse to honour the warranty if it is obvious that failure is due to incorrect memory being installed or that damage was caused by the person installing the memory. Dion Wired have probably chosen not to allow their staff to do any work on Sony products to prevent any warranty claim issues from arising as their in-store technical facilities are must not be Sony authorised service centres.
craniac's place 
The RAM slots in the laptops are pretty robust these days and it’s pretty damn difficult to insert one incorrectly so you should be okay doing it yourself.
I’d be inclined to think that Rectron wont do the install themselves since you did not buy it from them – but it’s worth a try if you don’t want to do it yourself.
I have no problem with adding memory myself and have done so on more than a few occasions in both laptops and desktops. I’m going to see how it performs with 2GB and will add another 2GB if I think it needs it.