I like the Lowepro Flipside series (I have the 300 and 400 AW) - the opening is against your back, quite secure.
Having said that, I'm considering a Fastpack for an upcoming trip where I plan to bring my 17" MBP. Rather than the older type, I'd get a Fastpack DSLR Video 250 or 350. You can ignore the 'video' part. The new series offers three improvements, IMO - a tripod attachment, a rain cover, and the laptop section loads from the top vs. the side.
I'm debating between the 250 and the 350. It's always hard to judge online - I recommend taking your gear into a shop if possible. For example, the Flipside 300 isn't supposed to hold a gripped body, but it does. The Fastpack DSLR Video 250 states 'pro dSLR (without grip) and 24-70' but based on the stated depth a gripped body will fit - I just need to know if an attached 28-300L will fit, despite what the specs say, and if my 17" MBP will fit in the 'holds a 15" notebook' pocket (measurements say yes, reality may be different).