Web measurement company comScore just announced it has acquired mobile metrics startup M:Metrics. ComScore is already one of the best-known companies for measuring web traffic from PCs, and the deal signals its intention to move into the mobile market by combining its tools with M:Metrics’. The combined data will provide a comprehensive look at combined Internet traffic from PCs and mobile devices.

We reported yesterday that M:Metrics would be acquired by “a large company with international operations”, and we said the Seattle-based startup was asking for between $50 and $80 million. It turns out comScore agreed to pay $44.5 million cash and issue 50,000 options for purchase of comScore shares to unvested M:Metrics options holders.

As we noted, the deal — which follows The Nielsen Company’s acquisition of mobile research company Telephia last August — is all about mobile advertising, a field where much of the traffic data is conflicting and unreliable. If M:Metrics can add reliable mobile numbers to the mix, Reston, Va.-based comScore will come even closer to domination of the traffic data market.

M:Metrics co-founders Will Hodgman and Seamus McAteer are joining comScore management.