This could surely be done, and there is clearly a demand for a way to sync up OF with some kind of web-based list service, but the problem for the Omni folks is: Which service do they choose to support? We've had people on the forums mention remember the milk and several other todo list webapps as well.
I don't think it's really in Omni's best interests to sync up with any one specific web-based list service, but there might be a way to enable easy plugin authoring. For instance, provide a robust Applescript interface to OF's export options (which have yet to be included in the alpha, but are said to be forthcoming) and that would allow individual authors to write a plugin-like set of scripts for syncing OF to whatever service they liked (this would be probably be more feasible in Leopard once the Perl, Python & Ruby APIs for Applescript is included in the standard OS). Maybe even provide an applescript interface to commonly needed syncing tasks, like an applescript function specifically for replacing one copy of an action with another.
With a robust enough interface, two-way syncing between OF and any number of online web services could be completed fairly easily and quickly without the Omni folks themselves having to really handle any of the service-specific code.
Maybe not a 1.0 feature, but something worth considering.