The same thing just happened to me. I just got an iPod touch a few weeks ago and within the first few days, the date and time started changing at random-- in fact, mine kept resetting to January 1970. The date and time on my laptop (2005 Apple iMac G4 laptop) are set correctly too. Each time it changes, I reset the date and time back to the current date and time and make sure the time zone is correct, but sometimes within seconds and other times after a few minutes or hours, it changes back to 1970. It seems like after I use an application for more than 30 seconds, it reverts to 1970. I know that sometimes apple products reset to 1969-1970 when the battery dies (the internal clock can no longer keep current time without battery power), but this isn't the case for my iPod touch; the battery was working the entire time.
It is quite an annoyance because when the date is set to 1970, it causes applications to act batty-- they either crash, are very slow to open, or don't function correctly. It's more than just wanting the time to be correct-- I wouldn't care, if it wasn't for the fact that applications don't work well if the time is incorrect.
I updated the system on the iPod Touch to the newer 4 version right after I got it. I thought my iTunes was up to date, but this morning I realized that there was a newer version available, so I updated that. I also followed what others who had written in forums had said about syncing my iPod to the computer again after waking the computer up. I thought maybe it would have solved the problem-- but after syncing and unplugging the ipod from the computer, I opened up an application and used it for about 5 minutes. After closing the application, I saw that the time had reverted back to 1970 yet again! (FRUSTRATING!) Anyway, I got rid of that application (an airhockey game). The time stayed current for about 3 hours, but the time changed back to 1970 just a minute ago.
I really have no idea what is causing the problem or how to fix it exactly. I'll try the total restore as the previous posters suggested. If that doesn't work, I'm going to take it to the Apple store to see if they can fix it.