We had Eclipse 3.4.1 64bit version with JRE 1.6.0_11 64bit and installed MyEclipse 7.0 through Update URL. It turns out that 7.0 has dependency on win32 version of some Eclipse bundles:
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Unsatisfied dependency: [com.genuitec.myeclipse.enterprise.workbench.feature.group 7.0.0.zmyeclipse70020081206] requiredCapability: org.eclipse.equinox.p2.iu/org.mozilla.xulrunner.win32.win32.x86/[1.8.1.z660_3-20070404,1.8.1.z660_3-20070404]