  We consider some consequences of describing the gauge and matter degrees of
freedom in our universe by open strings, as suggested by the braneworld
scenario. We focus on the geometric effects described by the open string metric
and investigate their observational implications. The causal structure of
spacetime on the brane is altered; it is described not by the usual metric
$g_{\mu\nu}$, but instead by the open string metric, that incorporates the
electromagnetic background, $G_{\mu\nu} = g_{\mu\nu} - (2\pi \alpha^\prime)^2
(F^2)_{\mu\nu}$. The speed of light is now slower when propagating along
directions transverse to electromagnetic fields or an NS-NS two form, so that
Lorentz invariance is explicitly broken. A generalized equivalence principle
guarantees that the propagation of {\it all} particles, not just photons, (with
the exception of gravitons) is slower in these transverse directions. We
describe a variety of astrophysical and laboratory-based experiments designed
to detect the predicted variations in the causal structure. We show that
current technology cannot probe beyond open string lengths of $10^{-13}$ cm,
corresponding to MeV string scales. We also point out that in a braneworld
scenario, constraints on large scale electromagnetic fields together with a
modest phenomenological bound on the NS-NS two-form naturally lead to a bound
on the scale of canonical noncommutativity that is two orders of magnitude
below the string length. By invoking theoretical constraints on the NS-NS
two-form this bound can be improved to give an extremely strong bound on the
noncommutative scale well below the Planck length, $\sqrt{|\theta|_{max}} <
10^{-35} {\rm cm} \times ({{\rm TeV} \over {\rm string} {\rm scale}}
