I think what you are describing is what happened to my iPhone. I was updating my software then after it was 'done' it just started flashing the apple logo every 5 secs. What I did was I connected my iPhone to my computer via USB then I pressed and held both the top and the home button for about 15 secs then released the top button while continuing to hold the home button. Then I followed these instructions: http://support.apple.com/kb/HT1808
Then, in my case it said that it could not restore my iPhone. If that happens to you, too, I suggest you follow these steps: http://support.apple.com/kb/TS1275
I hope it works for you, too.