Francis,
At 10.0, if your mxd is in page layout view, the mxd.activeView returns "PAGE_LAYOUT" (and not the active data frame within the layout). You would need to find the data frame by name (or loop through each df) and return the rotation accordingly.
If you are in data view, the mxd.activeView returns the active dataframe. At that point you can get the df.rotation value.
At 10.1 we plan to introduce a new MapDocument property called ActiveDataFrame which will be r/o and will return a reference to a data frame object regardless of data view or layout view.
Jeff