Rifkin and Mustard of Brown University also suggest that there are concentrations in Tharsis and Arabia:

GLOBAL DISTRIBUTION OF UNIQUE SURFACE PROCESSES IMAGED BY THE MARS ORBITAL
CAMERA.
http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1698.pdf

They viewed 8,000 MGS images for dark streaks and other features and found 160 images or 1.6% with dark streaks. Note in Fig. 4 where areas with dark streaks are indicated with red dots there are wide areas between Tharsis and Arabia with NO dark streaks (you should keep in mind as Jill England noted there is a mislabeling of the longitude in this figure.) That they were looking globally on Mars for several different geological features suggests that these 8,000 images would give a good indication of their relative distribution. It doesn't seem likely that dark streaks in areas of this size would be missed among these 8,000 images, considering their large number in Tharsis and Arabia. It is also notable that in the areas at the higher northern latitudes where they saw indications of another feature they were looking for, dust devil tracks, they saw NO dark streaks.
It's not clear to me the reason why you would expect the dust landslides to be concentrated near the equator. Certainly in the northern plains there is a dust covering as well, as Viking Lander 2 confirmed.
They also note that this concentration was also noted by Lucchitta and Ferguson:

"The process creating the landslide streaks is thought
The dark slope streaks are concentrated in the
high albedo regions of Tharsis and Arabia (Figure 4).
The distribution is consistent with that described
previously [11] though this is the first published map.
The interpretation of a disturbance of a near surface
layer of dust seems reasonable given the strong
association with bright regions and areas of dust
deposition."
GLOBAL DISTRIBUTION OF UNIQUE SURFACE PROCESSES IMAGED BY THE MARS ORBITAL
CAMERA, p. 1.

Reference [11] here is the Lucchitta and Ferguson report:

Dark streaks on talus slopes, Mars
In NASA. Washington Rept. of Planetary Geol. Programs p 188-190 (SEE
N84-23431 13-91)
http://adsabs.harvard.edu/cgi-bin/bib_query?1984pggp.rept..188F
[abstract]

Again it is not clear to me with dust depostion all over Mars as well as high albedo areas you would expect these dust landslides to be concentrated in Tharsis and Arabia.
Note also the Lucchitta and Ferguson work was based on Viking orbiter images which have a much larger surface coverage than the MGS images. Yet they still noted a concentration in Tharsis and Arabia.
A report given at the 1991 LPSC conference that references Lucchitta and Ferguson suggests their nearness to the equator is due to Viking orbiter images having better resolution at the equator:

Dark Talus Streaks on Mars are Similar to Aeolian Dark Streaks
Abstracts of the Lunar and Planetary Science Conference, volume 22, page 1509, (1991)
http://adsabs.harvard.edu/cgi-bin/bib_query?1991LPI....22.1509W

However, this would still not explain their concentration in Tharsis and Arabia.

Bob Clark