A pilot departing TIW VFR had a near miss with an unreported aircraft on a left downwind entry at about 1;000 FT.

Narrative

[I was] cleared for takeoff; northwest bound departure; Runway 35. One passenger; a rated private pilot was on board. [I] heard Tower communicate with a helicopter northwest of airport reported altitude 1;000 FT asking for clearance to transit airspace at 1;000 FT along water's edge. I believe there was another aircraft entering a (non-standard) left downwind to Runway 35 at that time also. At approximately 1;000 FT MSL; with no traffic in sight; my aircraft commenced an uncommanded roll to the left; which I immediately corrected. I said to my passenger 'What was that?' and he suggested it must have been wake turbulence; possibly from the left downwind aircraft. That seemed unlikely as his distance was quite a ways out. At that moment there was a radio report to the Tower saying 'Tower; I've just had a near-miss'. I did not catch the reporting aircraft tail numbers. Further; the reporting aircraft said 'I did not catch the tail number of the aircraft...' Tacoma Tower asked him 'What was the aircraft type?' and he replied 'A small single engine; on upwind.....' Tacoma Tower made no attempt to contact me; nor did I make any attempt to contact them; nor did the Tower respond to the reporting aircraft with any additional comments; as far as I could tell. I monitored TIW Tower frequency until I had proceeded well outside Class D airspace heading for my destination. During the period after this event I confirmed that my aircraft had been transmitting a Mode C VFR signal and confirmed with my passenger that at no time did either of us see any other aircraft; either before or after the event; other than the aircraft on left downwind at a receding distance of more than two miles.