Publicly sharing an iCal calendar to allow web viewing
In iCal, click the "Calendars" button in the upper-left corner, just under the traffic light. Right-click on the calendar you want to share, then select "Share Calendar..." This will open a new dialog, allowing the choice you mentioned between sharing read-only with everyone, or editable with invitees; make your selection and hit "Share".

Once the calendar is shared, again click the Calendar button and right-click the calendar you just shared, this time selecting "Copy URL to Clipboard".

(There's also a "Stop Sharing" option, in case you ever change your mind.)