Need a lighter backpack

If you want to use that pad and still get a frameless pack I believe the ULA packs will work. You fold up the pad in thirds (I think) and then slide it into a sleeve type system against your back, inside of the pack.

Here's a video describing one of their packs, including using pads as a frame.

http://www.youtube.com/watch?v=Un9Xwexwc4Q

My choice would be an ULA Circuit, but for packs from the big boys, the Ospreys aren't the best internal frame choice, they're the only choice. At least for packs close to 2 lb. with a frame.

The Six Moon Designs packs (Starlight, Traveler and the vanishing Comet), used with the "optional" (IMHO, mandatory) stays, weigh about 30 oz. and will easily support weights up to 35 lbs. I have a Comet and love it! I also use an inflatable pad. The main purpose of a pad in these packs, when used with the stays, is for padding, not support.

Of course pack fit is almost as individual as shoe fit, but this gives you another option to try.

New REI packs. Reviewed here recently sounded OK. I looked at them at REI last week, they seem well made, versatile in terms of how they can be configured… More affordable than the Offsprey, but a bit heavier. Great return policy.

The frame in the ULA Ohm eliminates the need for using a pad for structural integrity. I'm finding it comfortable with no pad at all just by packing the sleeping bag and clothing against my back. But I'll probably end up with an 8×20" piece of foam which will double as a sit pad. It's a beautiful pack, with great attention to detail.

If the Ohm is not enough volume, the ULA Circuit is a fantastic, but heavier, option. The Circuit has an optional aluminum stay.

I believe Brian won't take returns if you've used it (unlike REI). But I suspect you could wear it around the block or house for a couple weeks and return it in new condition if you don't like it. And if you take it on a trip and don't like it, then you can sell it on the Gear Swap forum.

Can't say enough good things about ULA packs and customer service.

Amy, Palo Alto

another ULA Circuit vote. It is not the lightest pack out there but it makes a 20lbs load feel like nothing.

Osprey is worth a look. Hands down these are the most comfortable I have ever carried. They have a 100% fix it policy even if it was eaten by a bear. The prices are low and they are sold everywhere. The Talon's are nice and small. I am currently living out of my Talon 33 I will write a review after I have 90 days under my belt. Ali

"Are there other internal frame designs worth considering?"

I'd highly recommend at least investigating the Ohm or Circuit, depending on your volume and weight requirements. The Ohm will carry up to ~28# comfortably(up to ~33# relatively comfortably) and the Circuit ~35#, IME.
The quality of Brian's workmanship is excellent and, with reasonable care, either will last you many years.