I've been posting a lot here about iPhone/IPod Touch/IPad apps, but I haven't mentioned the one app that actually has "decimal time" in it's name, Decimal Time Converter by CodeBurners (free). That is because it is a different type of decimal time.
This form of decimal time is actually widely used, mostly for calculating payrolls, and decimal hour time clocks are pretty common, that record tenths and hundredths of hours, rather than dividing the day by powers of ten. I have not found a good reference that describes this decimal time, although there are plenty of conversion charts and calculators for it. Rather than being used for time of day, standard hours are divided decimally in order do math with time intervals, without having to convert between the different standard time units, like hours, minutes and seconds.
What this particular app does is convert between standard hours and minutes (HH:MM), hours with decimal fractions (HH.HH), and minutes (MMMM). For instance, 689 hours and 40 minutes is 689.67 hours or 41360 minutes.
This form of decimal time is basically what was proposed by the French Commission de décimalisation du temps, led by Poincaré, in the 1890s, that standard hour be divided into 100 minutes, and each minute into 100 seconds. This proposal was abandoned, but at some point it became common in business.