We can cure many diseases nowadays, and thus the natural selection is very limited. Plus, mankind spent the whole beginning of its existence in almost the same hostile environment, and that's where he evolved the most, whereas our environment is changing very quickly. So, can we say that mankind froze, if not stopped, its own evolution ?
Not at all.
- Mutations accumulate independent of medical progress. In fact harmful mutations which would otherwise cause an individual to die can now be "cured" thus if anything increasing the gene pool.
- Typically medical progress extends our lives when we are much older, thus we have already passed mutations on to our children before a bottleneck in evolution could occur
- We are getting taller, less hairy and women are getting more attractive which is another form of evolution
- As new diseases develop the selective pressures on humans are ever changing. HIV which was never as widespread is now pushing certain immunogenetic traits to be favourable, the same is true for malaria (HLA 5701) and many others
In conclusion, whilst there are fewer and fewer strong selective pressures forcing extreme evolution, our bodies are designed to evolve slowly unlike viruses like HIV. Our mutation rates are around 1 in 10^9, allowing us to always slowly evolve as there will always be traits which are slightly more advantageous.
Keep in mind that most people live in environments where antibiotics are of limited availability and hygiene is not as rigorous as in the "western" world. Combine this with high population densities and you get, if anything, more natural selection due to the spread/proliferation of pathogens and the subsequent mortality of the patient. Also, don't confuse natural selection for evolution. Natural selection is one of only five causes of (micro-) evolution. Mutations will continue to occur. Sexual selection continues unabated, and migration is more important now than ever. The only evolutionary factor I'd argue is being diminished is genetic drift since there are so many people today.