There's some tricky things with the physics of sampling. By the laws of nature, you can only accurately represent frequencies up to one half of your sampling rate (the "Nyquist frequency"). This would be 22,050Hz at 44.1... which is fine for our hearing. Another goofy side effect of sampling is that signals above the Nyquist frequency can show up as "ghost" signals at half their actual frequency. To avoid this "aliasing", converters apply a low-pass filter (think of a high-cut) to strip the signal of anything above the Nyquist frequency.
The electronics to make good filters can be expensive, and it used to be worse only a few years back. The 48k converters were usually better because you could use a more gentle slope on the filters and still be able to cut all the way down to 20k (the practical hearing limit). The 44.1 converters had to have more aggressive converters to achieve the cut and avoid aliasing. A lot of guys that have used digital equipment professionally for a long time now, say that the difference in the converters has pretty much evaporated. If your target is 44.1, then it's fine to just stay at 44.1.
