Is it required and/or standard for second lieutenants to salute first
lieutenants ? I seem to recall a concept called 'comraderie of
lieutenants' which essentially means they treat each other as equals
except in formal situations.

According to Army Regulations (AR 600-25 "Salutes, Honors and Visits
of Courtesy"), Paragraph 1-3.(b.) “Salutes will be exchanged between
officers (commissioned and warrant) and between officers (commissioned
and warrant) and enlisted personnel.  Salutes will be exchanged with
personnel of the United States Army, the Navy, the Air Force, the
Marine Corps, and the Coast Guard entitled to the salute."

So the official answer is yes, it is required that 2nd Lieutenants
must salute 1st Lieutenants.  However, bear in mind that  "Military
courtesy is not a one-way street ... Mutual respect is a vital part of
military courtesy." (
http://www.uwosh.edu/departments/military_science/class/tsp_158-1181.html
 ) So, of course, the 1st Lieutenant must return the salute.

But AR 600-25 also states, in Paragraph 1-3.(g.) that "Local
commanders will carefully review saluting policies for their
installations. Where considered desirable in their judgment, they will
develop and publish modified saluting policies for congested, student,
or high density living areas where saluting would be highly
repetitious or otherwise infeasible."

And so the answer is also, no, it is not always standard, as you see
it can be  possible to adjust these regulations to suit local
situations.  According to an Army NCO of my acquaintance, in his
experience there does exist an "unofficial" tradition, as you suggest,
that Company Grade oficers (which includes Lieutenants and Captains)
do not necessarily need to salute one another unless at an official
function, nor do Field Grade Officers (Majors, Lt. Colonels and
Colonels), although Company Grade officers DO need to salute Field
Grade Officers and vice versa.

However, most experts and available knowledge sources emphasize that
"When in doubt about initiating or returning a salute, it is better to
salute than risk any transgression of courtesy." (
http://oep.osophs.dhhs.gov/dmat/resource/mil_courtesy_cond.html )

This is all a very complicated subject, which is covered under what's
known as Military Customs and Courtesies, and is spelled out in
excruciating detail, while still leaving room for local
interpretation, especially on an informal basis.

If you'd like to investigate the topic further, you can download a PDF
copy of AR 600-25 from
http://www.usapa.army.mil/searchtitle_number_pubs.asp

There is also information in the Field Manual of Drill and Ceremony –
Appendix A Saluting  at
http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/22-5/appa.htm

And the Army Promotion Board study guide has some information at 
http://www.armystudyguide.com/customs_courtesies/studyguide.htm


Search terms I used were:

Military Customs and Courtesies
AR 600-25
Army Regulations

I hope this has been helpful to you.

Byrd

Above and beyond. The particular distinctions among and between
company- and field-grade officers is particularly helpful.

byrd-ga
You forgot to mention US Public Health Service Commissioned Corps, one
of the seven Uniformed Services (USA, USAF, USN, USMC, USCG, USPHS CC,
and NOAA CC) when exchanging salutes between commissioned and warrant
officers.