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Back in early February there was a flurry of reports that Samsung were going to release a Galaxy S2 "Plus". Since then, nothing.
Anyone know what happened to this?

None of the above I believe.
I think the OP is referring to the rumours of the Samsung Galaxy S2 Plus, as it was dubbed, with 1.5Ghz processor and 4.5" display.
I think it turned out to be the Galaxy SII Skyrocket HD. See here:-
Samsung Galaxy S II Skyrocket HD - Full phone specifications