Mind you, no one is saying you can't get the source, just that you don't get it in the standard package. I'm sure if you point out the GPL they will be happy to supply the source of the Linux kernel and other GPL software that is included. They are, of course, under no obligation to supply they source of any of their own code, so long as is not derived from anything GPL.
However, as an ACER rep pointed out "the source code won't do you much good, since you don't get the root password".
In fact while it is possible to FTP into the machine and see the entire filesystem, and even log into the console by installing the flash card into a machine with a keyboard (you cannot telnet to the machine), you cannot modify anything other than what the web based interface allows. Among things that cannot be changed are the services that are started at bootup and the firewall rules.
Another thing you cannot do is install software. While I was told the machine in question was running the latest firmware, viewing the /proc/version file via FTP showed the kernel version as 2.2.11 - much older than the current stable kernel. You cannot upgrade the kernel yourself, you have to wait until ACER release a firmware upgrade to fix this and other potential security problems.
Given that one of the major uses of such a server is as an Internet gateway this is a major problem. Apart from the fact that you cannot yourself fix potential security problems, you don't even have the power to inspect your system to see if problems exist.
Linux is well know for it's stability and security - I'm sure this is one reason why ACER would have chosen Linux as their operating system. However without the ability to inspect and modify your system, the power to ensure your system's security is taken away. The GPL gives you the freedom to modify the software on your system, and as long as ACER deny you the right to do this they are breaking, if not the GPL licence, certainly the spirit of Free Software.
It should be pointed out that this does not apply just to ACER. Linux is quickly becoming the operating system of choice for appliance servers and embedded systems. Many Linux advocates see this as a good thing. But I hope that this popularity does not come with the cost of our freedom.