The chart below came to me from UC Boulder, and shows the number of yeast cells for various OD660 readings.
Cells OD660 x10^7 0.000 0.000 0.510 0.717 1.010 1.890 1.510 4.550 0.010 0.015 0.520 0.733 1.020 1.926 1.520 4.625 0.020 0.025 0.530 0.750 1.030 1.963 1.530 4.700 0.030 0.040 0.540 0.766 1.040 2.000 1.540 4.775 0.040 0.053 0.550 0.783 1.050 2.040 1.550 4.850 0.050 0.055 0.560 0.800 1.060 2.080 1.560 4.925 0.060 0.078 0.570 0.817 1.070 2.120 1.570 5.000 0.070 0.090 0.580 0.833 1.080 2.163 1.580 5.075 0.080 0.103 0.590 0.850 1.090 2.206 1.590 5.150 0.090 0.115 0.600 0.866 1.100 2.250 1.600 5.225 0.100 0.128 0.610 0.883 1.110 2.296 1.610 5.300 0.110 0.140 0.620 0.900 1.120 2.343 1.620 5.380 0.120 0.153 0.630 0.917 1.130 2.390 1.630 5.460 0.130 0.165 0.640 0.933 1.140 2.433 1.640 5.540 0.140 0.178 0.650 0.950 1.150 2.476 1.650 5.630 0.150 0.190 0.660 0.966 1.160 2.520 1.660 5.700 0.160 0.204 0.670 0.983 1.170 2.566 1.670 5.800 0.170 0.216 0.680 1.000 1.180 2.613 1.680 5.890 0.180 0.229 0.690 1.023 1.190 2.660 1.690 5.980 0.190 0.241 0.700 1.046 1.200 2.706 1.700 6.070 0.200 0.255 0.710 1.070 1.210 2.753 0.210 0.268 0.720 1.093 1.220 2.800 0.220 0.280 0.730 1.116 1.230 2.850 0.230 0.293 0.740 1.140 1.240 2.900 0.240 0.306 0.750 1.160 1.250 2.950 0.250 0.319 0.760 1.180 1.260 3.002 0.260 0.330 0.770 1.200 1.270 3.055 0.270 0.342 0.780 1.220 1.280 3.107 0.280 0.356 0.790 1.240 1.290 3.160 0.290 0.370 0.800 1.260 1.300 3.220 0.300 0.385 0.810 1.283 1.310 3.280 0.310 0.399 0.820 1.306 1.320 3.340 0.320 0.412 0.830 1.330 1.330 3.400 0.330 0.426 0.840 1.353 1.340 3.460 0.340 0.440 0.850 1.376 1.350 3.520 0.350 0.455 0.860 1.400 1.360 3.580 0.360 0.470 0.870 1.430 1.370 3.640 0.370 0.484 0.880 1.460 1.380 3.700 0.380 0.499 0.890 1.490 1.390 3.760 0.390 0.514 0.900 1.520 1.400 3.820 0.400 0.530 0.910 1.550 1.410 3.880 0.410 0.547 0.920 1.580 1.420 3.940 0.420 0.564 0.930 1.610 1.430 4.000 0.430 0.580 0.940 1.640 1.440 4.065 0.440 0.600 0.950 1.670 1.450 4.130 0.450 0.617 0.960 1.703 1.460 4.200 0.460 0.633 0.970 1.736 1.470 4.270 0.470 0.650 0.980 1.770 1.480 4.340 0.480 0.666 0.990 1.810 1.490 4.410 0.490 0.683 1.000 1.850 1.500 4.480 0.500 0.700
According to Current Protocols: OD600 of 1.0 is roughly 3 x 10^7 cells/ml.