How to detect and identify multiple points of contact
I answered the previous question thinking you didn't care which point was pressed and just wanted to know if any of them were.  

Now I see you want to know which points are pressed, and how to detect a large number using only a few microcontroller pins. My suggestion this time is to use some shift registers.
These only require a few pins and many can be chained together to read a lot of lines and turn the parallel data into serial data to be clocked into the microcontroller. You need an SR that does PISO (Parallel In, Serial Out) and can be cascaded. A universal SR like the CD74HCT299 should do okay. Have a look at Mouser, Farnell, Digikey, etc for more options - here are lots from Farnell.

Now you mentioned "simultaneous", but gave no definition of exactly how quickly you regard this to be. I'm assuming that if it's to detect human touch, then it doesn't need to be that quick compared to a microcontroller clock speed.

For instance, if you are reading 256 lines and your clock is at 1MHz, then you are looking at around 0.25ms to clock all the data in - this is easily fast enough to ensure you don't miss a touch. Depending on the shift registers used (propagation delay, maximum number of serially chained registers - e.g. you could do two or four chains instead of one long one) you could go faster - many small micros have an SPI peripheral which can do up to at least 8MHz.   

Depending on how you want to do things and how fast it needs to be, you could set it up as e.g a 8x8 or 16x16 matrix, scanning a row at a time.

Here is an example of two daisy chained SRs (from this page):