Both Yahoo and Google are free data sources and their quality are not 100% reliable.  Yahoo seems to be better.  Professional money manager usually don't use them and  buy the information from reliable sources such as bloomberg.
I checked 8/3/2012 data using my Schwab account and it is:
Open      Low        High        Close        Volume196.48   196.16   198.95   198.52       3,2,78,231
In general, they are good enough for doing some research and manage small amount of money. 
--- On Sat, 8/4/12, Doug Edmunds <dougedmunds@gmail.com> wrote:

From: Doug Edmunds <dougedmunds@gmail.com>
Subject: [R-SIG-Finance] Differences between data sources
To: r-sig-finance@r-project.org
Date: Saturday, August 4, 2012, 4:44 PM

Can someone explain why there are differences between Google and Yahoo in openings, lows, volume) are occurring, and which source is correct? (data downloaded 2012-08-04 2:40pm PDT)

> library("blotter")
Loading required package: xts
Loading required package: zoo

Attaching package: ‘zoo’

The following object(s) are masked from ‘package:base’:

    as.Date, as.Date.numeric

Loading required package: FinancialInstrument
Loading required package: quantmod
Loading required package: Defaults
Loading required package: TTR
> getSymbols("IBM", from="2012-08-01", src="yahoo")
[1] "IBM"
Warning message:
In download.file(paste(yahoo.URL, "s=", Symbols.name, "&a=", from.m,  :
  downloaded length 258 != reported length 200
> IBM
           IBM.Open IBM.High IBM.Low IBM.Close IBM.Volume IBM.Adjusted
2012-08-01   196.96   197.85  194.72    195.18    2559300       195.18
2012-08-02   194.16   196.60  193.02    194.45    2812600       194.45
2012-08-03   196.48   198.95  196.16    198.50    2668200       198.52
2012-08-03   196.48   198.95  196.16    198.52    3278100       198.52
> getSymbols("IBM", from="2012-08-01", src="google")
[1] "IBM"
Warning message:
In download.file(paste(google.URL, "q=", Symbols.name, "&startdate=",  :
  downloaded length 169 != reported length 200
> IBM
           IBM.Open IBM.High IBM.Low IBM.Close IBM.Volume
2012-08-01   196.96   197.85  194.72    195.18    2559365
2012-08-02   194.12   196.60  193.02    194.45    2812511
2012-08-03   196.73   198.95  196.18    198.52     736918

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