C182 pilot on IFR flight plan; reports NMAC with opposite direction C210 at 5;000 FT. Traffic call from ATC and NMAC occur simultaneously.

Narrative

On IFR flight plan from EMT; weather was clear with pretty much unrestricted visibility and marine layer just off southern California coast. Just received third vector after takeoff to join V186 westbound and climb to 5;000 (may have been 6;000). Was also instructed to change comm to next sector. I had to work with the HSI some as it was showing incorrect information; I had to rely on the digital readouts instead. While adjusting radios and flying to join the airway I looked up and noticed a Cessna 210 at my 12:00 going very quickly past my left side and slightly above my altitude. The plane was so close I could see its gray color and red or brown trim; and white wings. Just as I saw the plane and it passed; ATC issued a traffic alert. I was still on the old frequency; so thanks to the Controller for keeping an eye out. On the other hand; I'm wondering about the separation and why an alert was not issued sooner. For my part; I suppose you can never look outside the plane too much. Will now look into a.) reviewing my scanning technique; b.) maintain an active outside the plan perspective; especially when flying IFR in CAVU conditions; c.) investing in a PCAS device.