Samsung Galaxy S having internal sd card problems
I have the exact same problem with my Samsung Galaxy S.  You can also get it going again by holding the "On" button for about 15 seconds.  It has been happening now for about 2 months and it was only today that I happened to be looking at it as it hanged and I saw the message about the internal SD Card, before that I thought it was a corrupt operating system or faulty battery.  

The frustrating thing is that when it hangs I lose all my contacts, my text message history and my customised settings like shortcuts to certain apps on the desktop.  

It also loses the default setting for connecting to the Mobile Access Point set by my mobile provider. Sometimes when it reboots people can't ring me, they think it rings once but I don't hear it ringing at all and I get no missed called.  

It's funny, I notice that this post is dated December 27th and this is roughly when my phone started giving trouble, my phone is about a year and a half as well.  

The advice given here by Izzy is probably the best advice for this situation but I just wanted to contribute some content here that might help more people with the same problem to find this post because unless you are looking at the phone when it crashes you will never think that the internal SD Card was at fault.  It is more likely that you will find that your phone has switched itself off or has just hung.