Yes, the rain cover for Arkel's is very good (I use an Arkel Commuter).

For even more positive dryness, you could use a dry bag inside the Arkel BUG. Please note, MANY manufacturers make similar light weight dry bags / back pack liners (go to a backpacking store). However, I must say the Arkel's are competitively priced.

OR one of the tricks of ultralight backpacking is trash compactor bags as backpack liners. VERY tough and completely waterproof (though not sealable on top). One could easily find a perfectly sized trash compactor bag for the bug. Note this is kind of an 'update' of the plastic bag idea.

OR (anyone get the idea that I have lots of ideas) you can use a Ziploc Big Bag... these are great.