I don't think it is officially documented by Apple, mainly because it may change between iOS versions, but these are the figures stated in the "Learn iPhone and iPad cocos2d Game Development" book:
+-----------------------------------------------------------------+
| Installed Memory | Available Memory | Memory Warning Threshold |
+-----------------------------------------------------------------+
| 128 MB | 35-40 MB | 20-25 MB |
| 256 MB | 120-150 MB | 80-90 MB |
| 512 MB | 340-370 MB | 260-300 MB (estimated) |
+-----------------------------------------------------------------+