Please have in mind this is just mathematical calculation based on information I was able to find on
WIKI. Formula for theoretical calculating ppi is shown on attached image.
Let's start:
dp = SQUARE ROOT(320 pixels ^2)+(480 pixels ^2))
dp = SQUARE ROOT(102400+230400)
dp = SQUARE ROOT(332800)
dp = 576.8882040742385 pixels (hypotenuse of right triangle created by 320x480 screen resolution)
576.888 pixels / 3.2 inches = 180 pixels per inch
576.888 pixels / 3.1 inches = 186 pixels per inch
There you go: unofficial ppi measurement for T-Mobile G1 seems to comes out as 180 or 186 ppi.
So what you have is a conservative measurement of the pixels per inch.
We do not know for sure what is correct size of the screen (3.1 or 3.2) but here are both measures and ppi included.
It seems little bit to high for iPhone's only 164 ppi.
Here are some of the popular sizes of the electronics we use:
WideScreen monitors:
19" 1440x900 has 89 ppi
20.1" monitor 1680x1050 has 99ppi
22" monitor 1680x1050 has 90 ppi
24" 1920x1200 has 94 ppi
Mobile phones:
Nokia N95 3GB - 2.6" 240x320 has 153 ppi
TMOBILE MDA, TMOBILE WING and Nokia N95 8GB - 2.8" 240x320 has 143 ppi
N70 - 2.1" 176x208 has 130 ppi
E70 - 2.1" 352x416 has 259 ppi
N770, N800, N810 - 4.1" 800x480 has 225 ppi
[
source of this information is WIKI]
However, I do agree with DesignDawg that resolution is so low that you cannot really tell the difference and will be difficult for human eye to notice it.