While muscle soreness can be looked at an indirect indication that muscle growth is occuring, I don't think one can definitively say that a lack of soreness indicates a lack of growth, though it seems possible to me that the degree of pain is proportional to the degree of growth.
There are two current (related) mechanisms thought to cause delayed onset muscle soreness (DOMS) which follows exercise:
1. Tearing of muscle fibers leads to sensations of pain
2. Inflammation related to the body's mechanisms of rebuilding the torn muscle (vasodilation and increased blood flow due to histamine release, etc.) creates increased pressure which leads to pain.
This degree of pain and swelling is thought to be proportional to the degree to which the muscle was stressed during exercise, therefore if you are feeling less or no pain it may indicate that your muscles are no longer being taxed as greatly by the exercises you are doing.
One would expect that the greater the strain you put on a muscle the greater the growth, this is why training harder generally produces greater results. However, I don't know if we can really conclude that less pain means less growth. Until we do have a definitive answer, it is probably a wise idea to just keep pushing yourself as hard as you safely can (maintaining proper form and control) while also changing up your exercise routine regularly so that the same muscles are not worked in the same fashion every time.
Celticmusl, I'd be interested to read that article on protein loading. A state of positive nitrogen balance means that more nitrogen is being put into the body (through the intake of the amino acids which make up proteins) than is being used by the body to build proteins, fuel oxidative metabolism, etc. While a state of positive nitrogen balance would maximize the body's ability to rebuild the torn muscle filaments and thereby its ability to increase muscle mass, I do not see a direct relationship through which greater amino acid availability would reduce DOMS. It would be interesting to see what conclusions the author(s) reaches based on these findings.