Either may be used. "mL" seems to me preferred in the USA and "ml" il the rest of the world.

From reference:

"This unit and the symbol l were adopted by CIPM in 1879 (BIPM Proc.-Verb. Com. Int. Poids et Mesures, 1879,
p. 41). The alternative symbol, L, was adopted by the 16th CGPM (1979, Resolution 6) in order to avoid the risk
of confusion between the letter l and the number 1."

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Note added at 32 mins (2003-12-16 06:44:02 GMT)
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You wrote \"some texts use L and l intermittently\". This practice is definitely wrong in any language.

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Note added at 33 mins (2003-12-16 06:45:25 GMT)
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OOPS!

Please read: \"Either may be used. \"mL\" seems to me preferred in the USA and \"ml\" in the rest of the world.\"

is the term given in Chambers 21st. Century English dictionary, and it's the term I personally have always seen - and used. I would always have thought "mL" looked wrong.

6.1.2 Capitalization
Unit symbols are printed in lower-case letters except that:
(a)
the symbol or the first letter of the symbol is an upper-case letter when the name of the unit is derived from the name of a person; and

(b)
the recommended symbol for the liter in the United States is L [see Table 6, footnote (b)].
Examples: m (meter)
Pa (pascal) s (second)
lm (lumen) V (volt)
Wb (weber)

FROM: http://www.physics.nist.gov/Pubs/SP811/sec06.html

This is the general guide: Guide for the Use of the International System of Units (SI)
http://www.physics.nist.gov/Pubs/SP811/contents.html

Finally, http://www.physics.nist.gov/cuu/Units/index.html contains data such as:

Essentials of the SI
Introduction
SI units and prefixes
Units outside the SI
Rules and style conventions

Background
Definitions of the SI base units and their historical context
International aspects of the SI
Unit conversions







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Note added at 2003-12-16 12:57:48 (GMT)
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The site that gave link to the info in 6.1.2. above actually contradicts that info, saying that \" Litres, millilitres, and minutes are abbreviated using small letters, or symbols, and no period\" (http://www.aeg.org/geea_instruction.htm.)

The reason I answered this Q was that it came up among a few colleagues recently, and someone contacted an expert in the UK, who said the following: \"We use L (wherever possible as the base for concentrations), mL (in procedures etc) and dL. This prevents any confusion with the figure 1 or the unit \"1\". I believe it also conforms with an ISO recommendation\".

Opinions seem to vary, but in the interest of avoiding any confusion it seems that the capitalised form is probably advisable.

Investigating further:

Conventions of Usage in the S I
There are various rules laid down for the use of the SI and its units as well as some observations to be made that will help in its correct use.

Any unit may take only ONE prefix. For example \'millimillimetre\' is incorrect and should be written as \'micrometre\'.
Prefixes which make a unit bigger are written in capital letters (M G T etc.), but when they make a unit smaller then lower case (m n p etc.) is used. The one exception to this is kilo [k] to avoid any possible confusion with kelvin [K].
A unit which is named after a person is written all in lower case (newton, volt, pascal etc.) when named in full, but using a capital letter (N V P etc.) when abbreviated. *****An exception to this rule is the litre which, if written as a lower case \'l\' could be mistaken for a \'1\' (one) and so a capital \'L\' is allowed as an alternative.****
Units written in abbreviated form are never pluralised. So \'m\' could always be either \'metre\' or \'metres\'. \'ms\' would represent \'metre second\'.
To make numbers easier to read they may be divided into groups of 3 separated by spaces (or half-spaces) but NOT commas.
The SI preferred way of showing a decimal fraction is to use a comma (123,456) to separate the whole number from its fractional part. The practice of using a point, as is common in English-speaking countries, is acceptable providing only that the point is placed ON the line of the bottom edge of the numbers (123.456).
It will be noted that many units are eponymous, that is they are named after persons. This is always someone who was prominent in the early work done within the field in which the unit is used.
FROM: A Dictionary of Units at http://www.bioinfo.rpi.edu/~zukerm/dictunit.html.

that's what I've seen so far.
my 2 eurocents worth...

My experience has been that in medical/biochemical context, they usually use ml while in chemical and phsyics context they use mL.
I may be wrong, but this is my impression from having been in both the medical as well as the purely chemical (theoretical chemical) disciplines.