Although BirdieSync has been heavily tested, it is strongly recommended to backup your data before installing a new version.

To regularly backup your data, for instance you can use MozBackup for your Thunderbird profiles.

Note that you can also simply backup your Thunderbird/Sunbird directories:

XP:

Thunderbird C:\Documents and Settings\[users name]\Application Data\Thunderbird\Profiles\
Sunbird C:\Documents and Settings\[user name]\Application Data\Mozilla\Sunbird\Profiles\

XP: C:\Documents and Settings\[users name]\Application Data\Thunderbird\Profiles\
Vista: C:\Users\[user name]\AppData\Roaming\Thunderbird\Profiles\
(Application Data is a hidden folder: in explorer window, you must check the option "Show hidden files and folders")

You can also use a free online backup service. Some of them offer to backup up to 2Go for free. That can be enough to backup very sensitive data.

If you have a Pocket PC, you may use PPCPimBackup to backup all your PIM data.