I *think* I've found out why this happens! Why so many experience sync errors, or the cant establish a reliable data connection.
Turn off auto update the date and time and set manually.
I've found out after trying some things every time this happens that the date is moving around. Today loads of sync errors only to fine the date has shifted backwards by one year and one month on the wifes samsung spica i5700. When I corrected it, and got it working she then informs me that "every day the date when I use calendar changes around. Yesterday it was Dec 2007...."
Date changed, and contacts, calendar, gmail all sync'd correctly and I had access to the market place and was able to download apps again.
Hope this helps someone else having issues on their android device.