Current Airport Express Sample Rate?

Unfortunately, Apple still hasn't released any information on this. My best guess, at this time, is no.

For analog audio, the AirPort Express (AX) uses a 16-bit Texas Instruments PCM2705 DAC ... at least this is what was used in the 802.11g models. I do not have the DAC info on the 1st or 2nd generation 802.11n AXs.

For the AX, audio input is primarily provided by iTunes. Of course, it can also get input from other sources like Rogue Amoeba's Airfoil.

iTunes does most of the work. When iTunes plays back standard audio content (AAC, MP3, audiobooks, Internet music streams, etc., it decompresses those file formats and creates what's essentially a raw, uncompressed audio stream. That stream is compressed using Apple's Lossless Compression (via a QuickTime codec), encrypted, and then, streamed (via AirTunes/AirPlay) to the AX. The AX decrypts the stream, decodes it, and outputs it in either analog format or as a digital PCM stream. It is important to note that the AX functions only at a 44.1kHz sample rate. When you play 32kHz or 48kHz data, iTunes sample-rate-converts the data in real time before sending it to the AX. Also, the AX can only work with 16-bit data. You can attempt to stream higher data rate streams, but they will just end up being truncated before they can be played by the AX.

If iTunes is playing back a digital multichannel file format like AC3 (Dolby Digital) or DTS, those bitstreams (like the lossy formats) are wrapped in Apple's Lossless compression, encrypted, and then, streamed to the AX. In these cases, the AX would first decode the stream, and then, output the raw AC3 or DTS stream via its optical digital audio port.

Do you think it's actually a limitation of the Texas Instruments' DAC itself stopping higher bitrate support or just that Apple hasn't gotten around to adding it?

The AirPort Express, by design, was limited to what iTunes could stream. A number of folks got around this limitation by using an external DAC connected to the base station's digital portion of the audio port to basically bypass the internal DAC. Whether or not, Apple will add higher bit rates with future models is anyone's guess.

I'd primarily like to stream film mixes via Airplay and do away with long cabling runs in my studio space, but I can't do it if the best I can get out of the Airport Express/ Extreme is 44.1KHz 16 bit.

I find it hard to believe Apple considers this "good enough".

I couldn't agree more ... maybe Apple will provide an "audiophile" version of the Express in the not too distant future.