That's not a problem in GraphPad Prism.
I assume you have your data and you get vaguely sigmoidal shape of curve?...
(based on GraphPad v.4.x)
Analyze --> Curves & Regression --> Nonlinear regression --> Sigmoidal dose-response (variable slope)
tick on option: unknowns from standard curve
Now when you enter your unknowns in Y column under your standard curve data, you'll get new tab "Interpolated X values". Of course those results are in the same format and unit as your curve (eg. log of concentration). (You can do the re-calc in GraphPad: Analyze --> Data manipulation --> Transforms.)
Note one thing - this will calculate your results from anywhere on the curve so if you enter unknowns that are too far from linear range you'll get very strange results.