Being the resident Classical Violin Geek around these parts, here is my two cents:
As the very wise AlChuck mentioned, violin is impossible to sample, due to the articulations, vibrato, yada yada. The violin is the most expressive instrument, next to the human voice.
However, if you must use a synth, some ideas:
Machines like Emu Proteus and Kurzweil have decent sounds.
The trick is in the orchestration. Don't play anything short, or with a quick attack, or anything staccato or spiccatto (off the string fast notes). And never try to emulate a solo player, it wont work, always go for a string section sound.
I wouldnt play anything longer than a whole note, and since the beginning attack on most samples is slow, I would play as few notes as possible, just use it for backing pads.
Never play anything above an octave over middle C, always sounds fake. The best sounds are the lower ones in the cello range, or lower Viola range.
Make sure in your recordings to stick the violins way back in the mix if you are using synths. After all, most pop stuff uses strings to warm up the sound, so just a little goes a long way.