I recently attended a workshop on the potential effect of technology on
patient safety (http://www.pmpm-network.man.ac.uk/workshops_mar03.html). I
did a (somewhat superficial) literature survey for it on the effectiveness
of clinical decision support systems. There's more than 2 decades worth of
studies (some from AHRQ themselves), many of which report positive effects
of CDSS on patient outcomes whilst some say they had no effect. For example:

Initial diagnostic accuracy rose from 45.6% to 65.3%. The negative
laparotomy rate fell by almost half, as did the perforation rate among
patients with appendicitis (from 23.7% to 11.5%). The bad management error
rate fell from 0.9% to 0.2%, and the observed mortality fell by 22.0%. The
savings made were estimated as amounting to 278 laparotomies and 8,516 bed
nights.
Br Med J (Clin Res Ed) 1986 Sep 27;293(6550):800-4

Mothers receiving computer-generated reminders had 25% higher on-time
immunization rate for their infants
(Alemi, 1996)

Decision support system was safe and effective and improved the quality of
initiation and control of warfarin treatment by trainee doctors
(BMJ 1997;314:1252)

Computerized physician order-entry reduced adverse drug events by 55%
(Bates, 1998)

9% of redundant lab tests at a hospital could be eliminated using a
computerized system
(Bates, 1998)

74% of the studies of preventive healthcare reminder systems and 60% of the
evaluations of drug dosing models reported a positive impact
(Trowbridge & Weingarten, AHRQ, 2001)

No effect was found . on the management of asthma or angina in adults in
primary care
BMJ 2002; 325: 941-944

Decision support system did not confer any benefit in absolute risk
reduction or blood pressure control
BMJ 2000;320:686-690

Computerised decision support systems have great potential for primary care
but have largely failed to live up to their promise
BMJ 1999;319:1281

However, I don't know whether there has been any work looking at the impact
of other forms of IT. For example, whether an electronic appointments system
is 'better' for patients that a paper-based system.

Jeremy