Different iPhones have different published memory 4GB, 8GB and 16GB. The touch can have 32GB. My understanding is this is the off-line memory (disk alike).
How much actual fast ram is there in the device available for my Cocoa Application?
Is there a preconfigured virtual amount?
iOS doesn't support swapping memory pages to disk, therefore only RAM will be available to processes (however, it does support paging executables).
As Mehrdad Afshari says, it's 128Mb (except for the 3GS which has twice that), but why do you need to know? Not all of it is accessible to programs and the exact amount varies. Presumably activities like running iPod in the background saps some memory and OS2.1, for instance, seemed to leave less free memory than 2.0.
The key, application wise, is to cache stuff rather than require everything in memory and to heed the memory warning notifications. Your app will be forced to quit if you don't. (If it quits with status 101, you were using too much memory.)
iPhone and iPod touch has 128 megs of ram, but 3rd party applications should only count on having about 20 megs available.
Currently released Apple iOS devices have the following amounts of working memory:
iPhone 2G = 128MB
iPhone 3G = 128MB
iPhone 3GS = 256MB
iPhone 4 = 512MB
iPod Touch 1G = 128MB
iPod Touch 2G = 128MB
iPod Touch 3G = 256MB
iPad = 256MB