Ok, when you sync with .Mac, this data is stored on your "iDisk", in Library/Application Support/SyncSerices. You can see that in your online .Mac iDisk web browsing pane. But it's not in a form you can use. It's designed to be synchronized with other Macs.
On your own Mac, you can see the To Do items in Apple Mail, and in iCal (click the Thumbtack in the lower right).
If your email account is something besides .Mac (such as Gmail) and you set it for IMAP mode, you can see in your online email program a folder (or Gmail label) called Apple Mail To Do, and your To Do items are in there.
.Mac is designed to sync all kinds of data among your multiple Macs. It does that fantastically.
But if you want things to show up on the web, you have to sync with things that have web interfaces. Apple Mail works pretty well with Gmail, extremely well with Exchange 2007's IMAP support, and very well with a number of other IMAP based web services. With those services, Apple Mail creates folder that you can look at on the server with notes and to do items visible to you as though they were regular email items.
Alternatively, in iCal, you can publish a calendar including the To Do items, and publish it into your .Mac web page or .Mac web group, for you or your group of friends, to see. When you do that, the dialog tells you your web and calendar URLS:
The web view:
http://ical.mac.com/username/Your Calendar
A calender view: webcal://ical.mac.com/username/Your Calendar.ics
However, some of those calendars don't show To Do items, just regular appointments or events. So if you're using one of those web calendars, you'd want to make an iCal event instead of just a To Do.
If the web calendar you publish to doesn't have a place to see To Do items, you can easily make To Do items into events:
+Q: CAN I CHANGE AN EVENT INTO A TODO OR A TODO INTO AN EVENT?+
+A: Mac OS 10.3 or earlier: No.+
+A: Mac OS 10.4: To change an event into a ToDo, hold down the "Option" key and drag it into the ToDo list. To change a ToDo into an event, hold down the "Option" key and drag it onto the the date of your choice.+
If someone has an answer of an online iCal subscriber that can pull from the .Mac URLs when you publish to .Mac, and that online calendar shows the .Mac To Do items, give us a link. There's got to be some....