Wikipedia ain't enough for a reference.

http://dictionary.reference.com/search?q=per+...
Dictionary.com Unabridged (v 1.1) - Cite This Source
per mill /ˌpɜr ˈmɪl, pər/ Pronunciation Key - Show Spelled Pronunciation[pur mil, per] Pronunciation Key - Show IPA Pronunciation
–noun
per thousand.
Also, per mil.


http://www.bartleby.com/61/12/P0201250.html
per mil
VARIANT FORMS:also per mill
ADVERB:Per thousand.
ETYMOLOGY:per + (short for Latin mlle, thousand; see gheslo- in Appendix I).

http://encarta.msn.com/dictionary_1861725136/...
per mill
per mill or per mil
adverb
Definition:
in every thousand: in every thousand, or by the thousand

http://www.m-w.com/dictionary/per+mill
Main Entry: per mill
Pronunciation: (")p&r-'mil
Function: adverb
Etymology: per + Latin mille thousand
: per thousand
- per·mil·lage /(")p&r-'mi-lij/ noun

http://amsglossary.allenpress.com/glossary/se...
per mille (Symbol ‰.) Per thousand or 10−3; used in the same way as percent (%, per hundred, 10−2).
Per mille (by weight), is commonly used in oceanography for salinity and chlorinity; for example, a salinity of 0.03452 (or 3.452 percent), is commonly stated as 34.52 per mille.

http://mathworld.wolfram.com/Permil.html
Permil
The use of permil (a.k.a. parts per thousand) is a way of expressing ratios in terms of whole numbers. Given a ratio or fraction, it is converted to a permil-age by multiplying by 1000 and appending a "mil sign" %%. For example, if an investment grows from a number P==13.00 to a number A==22.50, then A is 22.50/13.00==1.7308 times as much as P, or 1730.8%%.

--
LEO:
related discussion: per mill / per thousand - promille

one-tenth of a percent [finan.] das Promille
per mill also: mil, mille - per thousand pro mille
per mill also: per mil pro Tausend

Google:
1,790,000 for "per mille" (Achtung: auch italienische Treffer)
724,000 for "per mille" -site:it
704,000 for "per mil"
104,000 for "permil"
64,600 for "per mill"
24,200 for "permille"


35,900 for "per mil" site:edu
11,600 for "permil" site:edu
701 for "per mill" site:edu
528 for "per mille" site:edu
201 for "permille" site:edu

Eher dagegen

Von allen Synonymen zum deutschen Promille scheint mir die "permille" das am wenigstens gebraeuchliche und das einzige, fuer das sich keine Woerterbuchbelege finden lassen. Nur im Zusammenhang mit "blood alcohol" scheint es eventuell etwas haeufiger gebraucht zu werden.

Eine Ergaenzung der exisiterenden LEO-Eintraege zu per mil und per mill um das deutsche Promille waere aber wohl unterstuetzenswert.

Permil scheint auch sehr haeufig zu sein, ist mir aber persoenlich unbekannt und ich finde auf die schnelle keine felsenfesten Belege.

Noun: permille
1. A proportion multiplied by 1000; one tenth of a percent
http://www.wordwebonline.com/en/PERMILLE


Percentage is the numerator of a fraction with a denominator of 100.
Millage or permille is the numerator of a fraction with a denominator of 1000.

Percentages are written with a percent sign (%) and permille are written with a permille sign (‰). Similar higher order fractions are parts per million (ppm), parts per billion (ppb), and parts per trillion (ppt).
http://www.andrews.edu/~calkins/math/webtexts...


During the last ice age, the isotopic composition of the ocean was enriched in d18O by about 1 permille, because sea level was lower and more water was trapped in the ice sheets. In order to compare the groundwater data for glacial and holocene periods, we need to correct for this effect in the source region and subtract 1 permille from the glacial d18O values.
http://www.ldeo.columbia.edu/~martins/isohydr...


Three dimensional movement related to distribution of salinity and temperature: Thermohaline convection.
Salinity is amount of salt in solution, measured in permille; example for salinity variations: Strait of Gibraltar (Fig. 7)
http://www.earth.rochester.edu/fehnlab/ees215...


Measuring parts of something, fraction, ratio, percent (permille, parts per million,
parts per billion).
http://64.233.183.104/search?q=cache:DrP66A19...


Other suggestions for improved legibility are:

* left-align the heading over the left-most column (called the stub);
* center-align heads and subheads over the column(s) they describe;
* indicate ranges with a dash not a slash or hyphen;
* put dollar signs (or other currency symbols), percentage (%) or permille (o/oo) symbols and units abbreviations only on the first and last lines and align them vertically;
http://www.eng.auburn.edu/~tplacek/courses/CH...

We express stable isotope
measurements as ratios of ratios (delta
values or δ), and expressed in permille (‰):
http://64.233.183.104/search?q=cache:3Yq3FRjo...

Cautious support.
It is obviously in use, but in a very "fachsprache" manner. It should maybe be tagged if it finds its way into LEO, but I am not quite sure how.

Zumindest zwei der .edu-Links fuehren auf Seiten von deutschen Expats. Zumindest hat einer an der Uni Muenchen promoviert und der fuehrt ein sehr unamerikanisches "habil.". :)))

Ich finde es schwer einen echten Nichtgebrauch nachzuweisen, aber bei mageren 200 Treffern auf .edu-Seiten im Vergleich zu 36 000 fuer "per mill" bin ich einfach sehr skeptisch.