When they find a bag of white powder they have probable cause to believe it might be cocaine (who carries flour in plastic bricks?) and it's also reasonable at that point that they would pursue a warrant for the home. But when they find a cell phone they have no reason to suspect the phone contains evidence of a crime. There should be a separate warrant for the phone. The phone has all your information on it, they can look through your emails using it -- to my mind that's like getting a free pass to search a home because someone was arrested for something unrelated (like a drunk and disorderly or something).

Actually my philosophy is that the prosecution is wildly out of line, charging criminals with way more than what is appropriate, so we need fewer laws and a stronger Fourth and Fifth amendments if anything. Suppose they find a black humor joke on your FB wall via your phone? Now you're charged with felony thoughtcrime and your bail is half a million dollars.

And in general, if you give the government enough rope they will start hanging everyone with it. For example, downloading PDFs or CSVs from a public server using a fake User-Agent or changing your MAC allows the government to pursue decades-long sentences under the CFAA. That's bullshit.

So I don't support searching the phone during an arrest, they should be required to get a separate warrant for that.