Will there be "tar" found in the particles?
The interstellar dust collector on Stardust had already found complex organic tar-like molecules with its CIDA mass spectrometer:

Tarlike macro-molecules detected in 'stardust'
MAX-PLANCK INSTITUTE NEWS RELEASE
Posted: April 29, 2000
"It is the size of these molecular fragments with nuclear masses of up to 2000 (water e.g. has 18 such units) which surprised us as much as the seemingly absence of any mineral constituents", explains Dr. Kissel of the Garching-based Max-Planck-Institut für extraterrestrische Physik. "Only organic molecules can reach those sizes". The largest molecules found in space so far are the polycyclic aromatic hydrocarbons (PAH) which reach masses of a few hundred mass units.
"The details of the mass spectra measured with CIDA show that the molecules of the interstellar dust must have about 10 percent of nitrogen and/or oxygen in addition to hydrogen and carbon. This means that these cannot be pure PAHs, which are planar, but are especially due to the nitrogen extend into all three spacial directions."
http://www.spaceflightnow.com/news/n0004/29tarstardust/

These "tar-like" particles are presumed to be interstellar because of their direction and high-speed. However, it is notable that the Giotto spacecraft also detected "tar-like" materials on the surface of comet Halley. Perhaps these detected particles arose from comets in other systems. Another possibility is that the CIDA detected particles that arose from jets from comets in our system. That could explain their high speeds and unexpected directions.
On Earth actual tar is formed from combustion or decay of living material:

Tar
http://en.wikipedia.org/wiki/Tar

La Brea Tar Pits.
http://en.wikipedia.org/wiki/La_Brea_Tar_Pits

Even when produced in industry you still need petroleum products or coal derivatives to produce it, which themselves arise from prior living material.
If the returned Stardust grains are found to contain true tar that would suggest they arose from past life in space.

c.f.,

Newsgroups: sci.astro, sci.astro.seti, rec.arts.sf.science, sci.bio.misc
From: Robert Clark <rgcl...@my-deja.com>
Date: 2000/04/29
Subject: Re: Tarlike macro-molecules detected in 'stardust' (Forwarded)
http://groups.google.com/group/sci.astro.seti/msg/f42528d44bba448a

Bob Clark