There has been much speculation that in 1949 Stalin urged Mao not to cross the Yangtze, but this seems to be based almost entirely on statements by Mao (and one by Zhou Enlai which may be simply echoing Mao) and it is contradicted by the bulk of the evidence. There is a good discussion of this in Dieter Heinzig, *The Soviet Union and Communist China, 1945-1950: The Arduous Road to an Alliance.*, pp. 170-174.
http://books.google.com/books?id=yCDv67C2BzkC&pg=PA170 For example, Mikoyan (according to his report on his meetings with Mao in Xibaipo) urged Mao to form a government as quickly as possible, "let us say, after the capture of Nanking or Shanghai"--which tacitly assumed Mao would cross the river. (p. 172) Moreover, Mao informed Stalin in mid-January 1949, two weeks before Mikoyan arrived in Xibaipo, of his plans to cross the Yangtze "already that summer." Stalin, who was asked by Mao to say something if he did not approve of any of Mao's statements, raised no objection in his reply or in subsequent telegrams (to the extent they have been published). (pp. 172-3) (In a long telegram in April on "military activities after crossing the Yangtze," Stalin advised the CCP to use caution in advancing to the borders of neighboring states; but the whole telegram is based on the premise that the CCP had already determined to cross the Yangtze, and Stalin did not indicate any disapproval of that decision.) Shi Zhe, who participated in the negotiations with Mikoyan, even claimed that Mikoyan expressly urged the CCP to "free all China completely." (p. 172) Ivan Kovalev, who witnessed the talks, was specifically asked if Mikoyan had asked Mao to stop at the Yangtze and replied that he did not recall Mikoyan doing so. Mao himself never claimed that Mikoyan urged him to stop at the Yangtze, so Mao must have based his assertion that Stalin urged the CCP to stop at the Yangtze on some interpretation or misinterpretation of some Soviet statement or conduct that remains unknown to us. (Also, curiously, when Mao presented Soviet ambassador Iudin in 1956 and 1958 with a long list of Stalin's sins vis-a-vis China--including his 1945 advice to the CCP to avoid civil war--he omitted any mention of Stalin or Mikoyan trying to stop the PLA from crossing the Yangtze in 1949.)
It is undoubtedly true that "Acting President" Lin Zongren (and some other KMT leaders) did hope that Stalin would pressure Mao into stopping at the Yangtze--and into agreeing to a de facto partition--but this seems to have been wishful thinking. "At first glance, statements by a historian from the People's Republic of China appear to support the thesis that Moscow had sought to restrain the PLA from crossing the Yangtze. He claims that Ambassador Roshchin had reached an agreement with President Li Zongren concerning three conditions for Soviet mediation, despite the Moscow declaration--reached under pressure from Mao Zedong--which stated that the USSR would not participate as a mediator in peace negotiations between the CCP and the KMT. He had thereby provided support for Li's attempt to 'set the boundary at the river'...and thus establish 'a Northern and a Southern Dynasty' [*bei nan chao*]. And yet the idea that Roshchin pursued a line that was different from Moscow's should be ruled out. Even if it were possible to find sources to provide evidence of Roshchin's alleged behavior, the resulting conclusion with respect to the *bei nan chao* question would go way beyond its target." (pp. 173-4)
It certainly seems plausible that Stalin would have preferred a North Chinese Communist state which would be weak (and therefore dependent on him) to a Maoist victory in all of China. But that he tried to impose division on China in 1949 remains doubtful--probably he knew that it would be futile to try to get Mao to stop halfway by this point.