Shares in the handset maker fell some 23 percent last week amid concerns about increased competition from Google’s (GOOG) Android operating system, which is being rolled out on a number of devices at a variety of carriers, including Palm partner Sprint (S).

This morning, analysts at Citigroup (C) cut their recommendations to sell from hold on Palm (PALM) while simultaneously raising their rating on Motorola (MOT) because of its “compelling” new Android handset, Droid.

“Motorola is launching of one of the most compelling offerings at [a] time when many investors have given up on the company’s handsets,” Citigroup analyst Jim Suva wrote in a research note.

“The revolution of product and application service offerings,” Suva added, “is going to start to crack open the enterprise door and could pose a risk for BlackBerry. Major shifts in promotion support creating a promotion commotion in the months ahead favor Motorola and post a challenge for RIMM and PALM.”

Over at CL King, analyst Lawrence Harris is similarly dubious of Palm’s prospects in the months ahead. Noting that Sprint executive David Owens said last week that the carrier plans to peddle a number of new Android devices from HTC next year, Harris sees unfavorable implications for Palm.

“Historically, Sprint has been Palm’s largest customer. Indeed, in the August quarter Sprint accounted for 85 percent of Palm’s revenues,” Harris wrote in a note to investors. “In FY09 (May) Sprint represented 43 percent of total Palm sales. Sprint has a U.S. exclusive on both the Pre and the Pixi through calendar year end. The Pixi will be launched at Sprint on November 15 for $99.99. The Pixi is fairly similar to the $149.99 Pre. Verizon has stated that it will begin offering the Pre in early CY10.”

Expounding on his analysis, Harris notes that “According to a report on Mobile Today, a U.K.-based publication, initial sales of the Pre through Telefonica’s O2 unit in the U.K. have been slow. If this report is correct, than the bulk of Palm’s sales over the next few months will probably continue to be generated through Sprint.”

That Sprint will soon add some slick new Android handsets to its lineup is worrisome, then, indeed. For Palm, it seems, driving conditions on the road to recovery are looking increasingly hazardous.