I just did a more thorough test and I reported the following to Omni:
Dear OmniFocus team,
I've just started to use iCal synchronizing, and I've noticed a bug.
If there is a colon in a task, and the task is altered in iCal (or on my Palm), the material before the colon is deleted.
Example: I create a new task with the name "New task: what happens to text before the colon?" in my Library context, in my default task bucket (called "Miscellany").
I then sync with iCal. In iCal the task shows up as follows: "@Library: New task: what happens to text before the colon? [Miscellany]"
Scenario A: I now check off that task. When I sync, the task is deleted from iCal as it should be, but it appears in OmniFocus, completed, as simply "what happens to text before the colon?"
Scenario B: I edit the text, e.g. by adding "fnord" before the question mark. When I sync, the task appears in OmniFocus as "what happens to text before the colon fnord?" In iCal it now appears as "@Library: what happens to text before the colon fnord?"
In either scenario, data are lost--everything before the colon. I discovered this when I had a task to pick up a book at the library. The author and title were before the colon, and the call number after it. I knew what had been there, but I could imagine a scenario in which the data that are lost might be irretrievable (other than by restoring from a backup, which I know all alpha testers keep religiously...).