Something is bothering holders of Flextronics (FLEX) shares today.

There’s no obvious reason; I find no major news or analyst calls. But it might have something to do with this morning’s announcement of results at Sony Ericsson (SNE, ERIC). The joint venture in the latest quarter was the single largest customer for Flextronics, accounting for more than 10% of revenue according to its most recent 10-Q. Sony Ericsson said it grew handset units 18% on a year-over-year basis in the fourth quarter. According to Marketwatch, Sony Ericsson indicated that it could be affected by a more difficult economic environment in 2008.

The Street seems split on the implication for FLEX from Sony Ericsson’s results. Credit Suisse’s William Stein write that the numbers are “a small negative for Flex.” He notes that shipments of 30.8 million units in the quarter were light of the consensus of 31.5 million. But Bank of America’s Jeff Walkenhorst finds the Sony-Ericsson news to be “a slight positive” for the company that could “assuage some investor concerns.”

Or maybe note. FLEX today is down 62 cents, or 5.7%, at $10.18.