  Long Gamma Ray Bursts hold the promise of probing star-formation and metal
enrichment up to very high redshifts. The present GRB samples with redshift
determination are largely incomplete and therefore a careful analysis of
selection effects is mandatory before any conclusion can be drawn from the
observed GRB redshift distribution. We study and compare three well defined
samples of long GRBs detected by Swift, HETE2 and BeppoSAX. We find that Swift
GRBs are slighly fainter and harder than BeppoSAX and HETE2 GRBs, as expected
due to the higher energy range in which Swift GRBs are detected and localized,
compared to BeppoSAX and HETE2. Gas and dust obscuration plays a role in
shaping the GRB samples and the present samples of GRBs with redshift. We argue
that the majority of the bright Swift GRBs without redshift might actually be
z<~2 events therefore the present Swift GRB sample with redshift is biased
against low-z GRBs. On the other hand, the detection of bright UV rest-frame
afterglows from high-z GRBs, and even from those with large X-ray obscuration,
implies a dust amount lower than in nearby GRBs,and/or a different dust
composition. If this is the case, the Swift sample of GRBs with redshifts is
probably a fair sample of the real high-z GRB population. The absence of high-z
GRBs in the BeppoSAX and HETE2 samples of GRBs with redshifts is probably due
to the fact at the time of BeppoSAX and HETE2 follow-up faint afterglows of
high redshift GRBs will have weaken below the spectroscopic capabilities of
even 10m class telescopes. The redshift distribution of a subsample of Swift
GRBs with distributions of peak-fluxes, X-ray obscuration and optical magnitude
at a fixed observing time similar to those of the BeppoSAX and HETE2 samples,
is roughly consistent with BeppoSAX+HETE2 redshift distribution.
