More workers use iPhone and Android smartphones combined than BlackBerry devices, according to a survey of 1,681 U.S.-based workers released today by Forrester Research.

That finding highlights what many have known for a while about the entrenched workplace smartphone veteran: the BlackBerry faces trouble from its competitors.

The BlackBerry, made by Research in Motion, still leads among U.S. workers, with 42%, the survey said, with Apple’s iPhone accounting for 22% and Android devices, 26%.

The survey also found that nearly half, or 48% of the group, said that they chose the primary smartphone used for their work without considering what their company supports. Only 29% said they chose the smartphone from a list of phones the company supports, while 23% said they had no choice in the matter.

Often, corporate IT shops will choose BlackBerry smartphones when requiring a worker to use a specific smartphone, partly because of the perceived security benefits, many analysts, including at Forrester, have found. The growth in Android phones and the iPhone — many of them brought to workplaces by workers independently — are forcing IT shops to rethink that decision, however.

Ted Schadler, a Forrester analyst, said the survey points to two major trends. The first is that more workers than ever are bringing consumer-focused devices, such as Android and iPhone smartphones, to use for work, and more companies are supporting those devices.

The other trend is that IT shops are not realizing that even if they require that employees use a BlackBerry or other device at work, those same workers often bring in a second phone of their own to do other kinds of work-related tasks with applications that are not supported on the company-required phone.