I'd be willing to bet good money that benchmarks are still going to hinge on the speed of your Ruby app and its database access. Apache, despite not being the "new, new thing" is perfectly capable of saturating pretty much whatever connection you have.
This is something I keep trying to tell people whenever micro-benchmarks appear showing that such-and-such web server handles >5k concurrent connections within a particular memory footprint, or that this-and-that message queue supports 8k enqueue/dequeue ops per second without hitting disk.
How many Rails apps out there can handle more than, say, 200-300 concurrent clients without either pegging the CPU or bringing the database to its knees?
Putting Passenger behind Nginx just removes a need to install + configure Apache for folks who are already use and like Nginx. It may also have some benefit on memory-constrained environments like entry-level VPS hosts.
That being said, I personally think that folks who spend days or weeks tweaking their deployment to run in <256MB of RAM to save a few bucks a month on hosting should probably reevaluate their priorities.
