Courtesy of the ?strptime help file (with the example changed to your value):
> z <- strptime("2010-01-15 13:55:23.975", "%Y-%m-%d %H:%M:%OS")
> z # prints without fractional seconds
[1] "2010-01-15 13:55:23 UTC"
> op <- options(digits.secs=3)
> z
[1] "2010-01-15 13:55:23.975 UTC"
> options(op) #reset options