Run "ps". You'll see about 70 - 100 tasks and threads running on a phone which is just booted and setting "idle". Just becase you cant run two apps on the screen at one time doesn't mean that it is not multitasking.
Running "cat /proc/meminfo" shows 367420kB of ram. This is what is made available to the kernel to pass out as it sees fit. There is also that pesky proprietary radio stuff running along side the kernel. It is the junk that handles talking to the cell phone towers and satellites and basically all the stuff that your phone can do that a computer cannot. That radio crap needs ram to do the stuff that it does, so there as a chunk taken away for that. Sure, they could have given you all 512MiB of ram to play angry birds and watch movies. But instead they chose to put silly things like gps, 3g internet, text messaging, and cellular phone service in.
The phone does, indeed, have 512MiB of ram, and it can all be accessed while running in download mode. Using the qualcomm memory debug program, you can dump the ebi0_csX.bin files. Each one of them is a 256MiB chunk of memory. Add the two together, and you'll hopefully get 512MiB.
