  A search for new phenomena in ttbar events with large missing transverse
momentum in proton-proton collisions at a center-of-mass energy of 7 TeV is
presented. The measurement is based on 1.04 fb^-1 of data collected with the
ATLAS detector at the LHC. The search is carried out in the single-lepton
channel, characterized by an isolated lepton of high transverse momentum, four
or more jets and large missing transverse momentum. Contributions to this final
state may arise from a number of Standard Model extensions. The results are
interpreted in terms of a model where new top-quark partners are pair-produced
and each decay to an on-shell top (or antitop) quark and a long-lived
undetected neutral particle. The data are found to be consistent with Standard
Model expectations. A limit at 95% confidence level is set excluding a
cross-section times branching ratio of 1.1 pb for a top-partner mass of 420 GeV
and a neutral particle mass less than 10 GeV. In a model of exotic fourth
generation quarks, top-partner masses are excluded up to 420 GeV and neutral
particle masses up to 140 GeV.
