I had this same bug problem and 5.2 did not fixed it.
Further investigation lead to IsAllDay in arguments always returned False no matterwhat I set it to in my datasource.
I'm using IE 7 and SQL 2008
The work around I used was I added it to the TAGS
1)
SELECT [id],[eventstart] as [start], [eventend] as [end], [name],[resource] as [column], [allday],[allday] as islocked,
2)
Added islocked to property tags
3)
Then I could access it in in the cell beforerender method mention above.
e.Tags["islocked"] return string "True" or "False"
Hope that helps others.