TBM8; IFR inbound to APA; began descent and altered pattern before appropriate approach clearance was issued.

Narrative

I was being vectored by DEN Approach to land at APA. Approach issued this clearance: 'Descend and maintain 8;000 FT; contact Tower;' which I read back. Then it occurred to me that I hadn't heard 'Cleared visual approach;' which I expected in the circumstance; and been told to expect. I called Approach to ask if I was cleared for the visual approach. The frequency was congested and I got no response. I decided to switch to Tower frequency and report. The Tower frequency was also congested. I was still in the descent to 8;000 FT; and turned toward a base to land thinking I was on a visual approach. I got the Tower a few seconds later; and he told me to turn southbound and that he would call my base. A few seconds after that; after I had just started the turn southbound; he called and told me to continue on base and that I was cleared to land. I landed uneventfully. Upon reflection; I realized that I had not been cleared for the visual approach; and so my turn to base was not authorized. I was confused by not having been cleared for the visual; because in all of my flying experience I had never been handed to Tower by Approach without an approach clearance of some sort. As it was; I was on a vector for downwind; near the approach end of the active runway; and the circumstance suggested an imminent clearance for the visual approach; which I had been told to expect. I would have to speculate as to the reason for this deviation from all of my previous experience; and why Approach would hand me to Tower with no approach clearance. But; that is what I think happened; and I don't believe I had been cleared for a visual approach. This was reinforced by Tower's request for me to turn southbound (back to downwind). I should have 1) persisted in my attempt to get clarification as to my clearance from Approach; and 2) remained on the downwind vector until either I received that clarification or made contact with Tower.