It’s probably fair to assume that at least part of the Q1 2011 bump in the blue line at right can be attributed to the Feb. 10 launch of the Apple (AAPL) iPhone for Verizon Wireless (VZ).

As Piper Jaffray’s Christopher Larson notes in a report to clients Monday, 38% of the new cellular subscriptions last quarter went to Verizon, 11 percentage points ahead of AT&T (T), its nearest competitor.

AT&T had used the iPhone to cut into Verizon’s lead for more than three years, although those days were over — thanks to heavy marketing for Verizon’s Google (GOOG) Android phones — even before AT&T’s exclusivity deal with Apple ran out.

Still, says Larson, things could have been worse for AT&T.

“Despite losing share,” he writes, “we think it is important to note that AT&T only lost the same percentage point of share as Sprint and T-Mobile despite the significant headwind of losing iPhone exclusivity. Due to its larger size, 1 percentage point of share loss also hurt AT&T less.”

Below: The same information shown as gross postpaid new subs, and the data from which both charts were drawn.