Corporate bond spreads or the difference between investment grade (Aaa rated) and high yield (Bbb rated) are currently 80 basis points. They last reached these low levels in October 2007 at 79 basis points.
When this spread comes down to such low levels it signals an overbought corporate bond market as high yield debt no longer lives up to its name. Similar to the "dash for trash" in equity tops.
Bbb rated corporate debt yields peaked in October 2008 at 9.49% and now yield 5.83% (a 38.6% drop in yield). The following chart shows the inverse relationship between Aaa / Bbb spreads and the SPX.