UnitedHealth Group (NYSE:UNH) said today that it agreed to acquire Sierra Health Services, Inc. (NYSE:SIE) for approximately $2.6 billion, giving the health insurer a larger marketshare in the quickly expanding Southwest region and boosting its senior health-care capabilities. Sierra shares rose over 15% on the news, while UnitedHealth shares moved up fractionally.

Boston Scientific Corp. (NYSE:BSX) said its Board of Directors has authorized management to explore an initial public offering of a minority stake in its endosurgery group. The medical device maker said an IPO would involve selling approximately 20% of the endosurgery group, which would remain a majority owned subsidiary.

Wireless Facilities Inc. (NDAQ:WFII) said it plans to delay its 2006 annual report as it continues to investigate possible options backdating. The company is expected to record a $9.2 million charge related to the accelerated vesting of all of its employee stock options, an $18.3 million goodwill impairment charge, and a $3.4 million asset impairment charge.

Several changes were announced to the S&P500 index. Host Hotels and Resorts (HST) will be added to the index on a date to be determined, replacing Phelps Dodge Corp. (PD), which is being acquired by Freeport-MyMoRan Copper & Gold Inc. (FCX). Also, the S&P said it would be adding Option Care Inc. (OPTN) to the SmallCap 600 index after the close of trading March 15th. This company will replace Hancock Fabrics, inc. (HKF).

Cosi Inc. (COSI) said that its Chief Executive Kevin Armstrong has resigned for health reasons. The company named Robert Merritt, one of its directors, as interim CEO, effective immediately.

Netease Com Inc. (NDAQ:NTSE) said that its Board of Directors OK'ed a new buyback program that would enable the company to repurchase up to $100 million ADRs.

Merck & Co., Inc. (NYSE:MRK) said that it plans to appeal the Humeston Vioxx verdict, where a N.J. jury awarded $20 million to plaintiffs.

Texas Instruments Incorporated (NYSE:TXN) said that its inventory correction was "winding down" while its customer inventory levels were improving. Consequently, the company expects growth to resume again in Q2.