any laptop has spdif output?

Not that I'm aware of. Best you can do is a mac with toslink out.

MacBooks has spdif output. You would still be better off with a quality USB->SPDIF converter such as Wavelink, Off Ramp, etc.

Some PC laptops do, sure. For example, I have seen a Dell Inspiron (1720, other models may have similar) that could take an optional ($35 or so) multimedia cable header that included a coaxial S/PDIF RCA output. This hooked up to a built-in multifunction port on the laptop and used the laptop's internal sound hardware.

There are also external sound cards that attach via USB and provide digital outputs.

HDMI can also be used for digital audio output, and is increasingly common on newer laptops.

As for PC laptops that have a toslink or RCA S/PDIF output actually built in to them (without an external cable header/adapter), that I haven't personally seen. I am not a MAC guy, so that may be something you can find on the other side of the fence.

Are any of these good solutions? That's up to you to decide, but there are quite a few ways to get digital audio out of a PC laptop.

Also, isn't the limiatation you speak of a limitation of the USB DAC, and not the PC?

The easiest thing to do is to get a USB-to-S/PDIF converter. Both the Halide Bridge and the Musical Fidelity converter recently got a big thumbsup in Stereophile and TAS and are relatively inexpensive. They also support USB asynchronous transfer mode where the PC is slaved to the converter's clock which reduces jitter significantly. You will only need a converter if your DAC does not have a USB input or if the USB input as you say is limited to supporting 16/44 file formats. There are indeed quite a few USB DACs that support hi-rez file formats that you can connect your laptop to directly via the USB port. Check out Computer Audiophile (http://www.computeraudiophile.com) and The Well-Tempered Computer (http://thewelltemperedcomputer.com/HW/Connect/index_connect.htm). Hope this helps.