Directly, this is not possible at this time. Apple does not allow third party software to integrate with the native calendar (and the iTunes sync).
However, the SyncML client built into Todo+Cal+Sync allows synchronizing your data with any SyncML compatible server, of which a growing number are available. Wikipedia has a pretty complete list of them in their SyncML article.
For desktop synchronisation, a product from Bertels+Hirsch named Winfonie mobile provides a SyncML server for Windows PCs that interfaces with Outlook, Lotus, Tobit and other software.
On the internet, for example with the service GooSync.com you could sync Todo+Cal+Sync with Google calendar, and then use one of the solutions to synchronize your Outlook or iCal with Google as well. This way, you get indirect sync between your iPhone and your desktop, even if iTunes sync is not possible - in exchange, you gain mobility: you don't need to connect the iPhone to your Mac or PC to synchronize using SyncML.
Another service that offers sync via SyncML, and also connects to a number of other calendaring system is ScheduleWorld.com. The Wikipedia article mentioned above has many more services listed. Please also note that some mobile carriers provide a SyncML service as part of their offering to mobile users, so it might be worth checking with your mobile service provider. | 
