iPhone OS 3.1.3 was released earlier this month which brought some important security and bug fixes.
iPhone OS 3.2 SDK for iPad was released last month. This version of OS does not support iPhone and iPod touch devices. It runs only on iPad.iPhone OS 4.0 is expected to be released in the last week of March to go concurrently with iPad.
Next-gen iPhone 4G however will most probably be unveiled in June at this year’s WWDC conference.