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It depends on what you mean by "modern". Are sleigh beds not modern enough? They have clean graceful lines and most will accept box-springs...

Instead of looking at big chains, try looking at antique stores that specialize in 20th century furniture, you'll probably find plenty of beds. Unless it's a King size, as that is a relatively new size. I've found that a high quality old piece can often be had for the same or less than a new, mass produced thing. Try shops like Apartment or x-21, there are others right in the vicinity there on valencia st.

I bought a Valencia sleigh bed from Pottery Barn after a long search identical to yours. Beds that take both box spring and mattress are just insanely hard to find and I hate that the platform bed thing has completely taken over at every store.

I love my bed, it's very high up but I like climbing into bed.

I too love climbing into my big cozy sleigh bed! I got mine from the now defunct Evolution Furniture outlet in Berkeley, but here is a link to Copeland Furniture in Vermont that makes it:
http://www.copelandfurniture.com/collection.php?id=1
I have the Sarah sleigh bed in maple and cherry, with georgous raised panels. It also comes in a shaker style spindal bed. For me, this is a timeless design of exceptional quality that I plan to have for years to come, and it works well with our European Sleepworks mattress and boxspring. No sense compromising good sleep for style!

I have the same problem, a McRoskey box-spring mattress looking for a modern bed. After a long reasearch I came to the conclusion that a Heywood-Wakefield is the best option. You can either find a vintage bed from Another Time in SF or have new bed custom made from you from Penny Lane in LA.
Still another option is to order a new bed from the company who is the new owner of the Heywood-Wakefield brand.

http://www.penny-lane.com/main.htm

http://www.heywood-wakefield.com

I think it's weird that the platform bed has taken over too, but the trend worked out well for me when I bought a bed and mattress recently. I was able to buy one of the McRoskey mattresses made for platform beds and only had to pay for the one piece instead of both mattress and box spring, which is great because I LOOOOOOVVE my mattress now.

I bought a platform bed at Room & Board which has slats. When McRoskey came to deliver the mattress, they said that the R&B bed wasn't made for their platform bed mattresses because of the slats. We ended up going out to Home Depot and having a sheet of melamine cut to fit into the well of the bed on top of the slats. So far it's been great.

Have you looked at Ethan Allen? I keep seeing things in the press about how they have some new "modern"/"urban" collections, and it looks nice from pics I've seen. They might have more of a blend of traditional features with the modern lines?