It’s a simple calculation if you take the iPhone 3GS as baseline, which has 256 MB of memory. If we look at the numbers of available memory on the iPhone 3GS we notice that the maximum free memory on the 3GS is close to 150 MB – if you want to be in the region where you don’t want to encounter memory warnings you need to be below about 120 MB.
I know that iPad developers reported iPad App rejections (no sources given to protect the innocent) when memory usage was between 120 to 150 MB. Unsurprisingly this is also in the range where you get memory warnings or crashes on the iPhone 3GS. That leaves me to conclude that the iPad has 256 MB of RAM and not 512 MB as stated on Wikipedia (without citation).
It’s still plenty to work with though even if you consider the larger framebuffers, 6 MB on the iPad compared to the 1.2 MB on the iPhone 3GS for two framebuffers. You have 5 MB less to work with compared to the 3GS, that’s at most 4% less available memory.
On a related matter – would you like to see the iPad taken apart piece by piece? They conclude by the memory chips that the iPad does have 512 MB by decoding the chip numbers. I’m not so sure about that – it’s two 2 Gigabit chips apparently making 512 MB but then the question is: why do we developers see the same free amount of memory as on the iPhone 3GS? Why and how does the iPad keep the remaining 256 MB to itself? I remain doubtful that it’s really 512 MB.
For us developers, the device seems to offer only the same free amount of memory than we have on the iPhone 3GS – so it doesn’t really matter whether the iPad has 512 MB or 256 MB RAM in hardware.
CONFIRMED: iFixit have since corrected themselves and now say that the iPad has indeed 256 MB of memory.