Why isn't the silicon analogue of benzene flat?
The linked article is a short summary of a Science article: "A Tricyclic Aromatic Isomer of Hexasilabenzene" by Kai Abersfelder, Andrew J. P. White, Henry S. Rzepa, and David Scheschkewitz; Science 2010: 327 pp. 564-566.

They propose a new term, "dismutational aromaticity":


  The highly dispersed $\ce{^29Si}$ nuclear magnetic resonance shifts in solution ranging from +125 to −90 parts per million indicate an inhomogeneous electron distribution due to the dismutation of formal oxidation numbers as compared with that of benzene. Theoretical analysis reveals nonetheless the cyclic delocalization of six mobile electrons of the $\pi$-, $\sigma$- and non-bonding type across the central four-membered ring. 


They point out that while $\ce{Si=Si}$ bonds were ignored for a long time, multiple compounds have been prepared, even including delocalization, like 1,2-disilabenzenes.

They prepared $\ce{Si6R6}$ with "Tip" or 2,4,6-triisopropylphenyl groups.


  The surprisingly stable 3a can be exposed to air for hours as a solid or for minutes in solution without detectable changes.


Long story short, there is delocalization around the central 4-center ring:


Note that there is a 1,2-shift occurring, and that 2 $\ce{Si}$ atoms have two substituents, 2 have one substituent, and 2 have no substituents.

The paper argues that a fully planar hexasilabenzene might be possible, but:


  the choice of precursor may be important in any synthetic approach.


Presumably the choice of substituent is also relevant to prevent the rearrangement here.