Careful about the "warrantless" here. It's true, it's a warrantless search, and don't think I'm saying that's an illegitimate concern. However: The police already have broad authority to search you and your immediate person incident to an arrest. If this is news to you, you really want to read up on it. This is one of the big things that makes a formal arrest different from mere detention, and one of the reasons the…
The search incident to lawful arrest doctrine is rooted in officer safety. Specifically, the Court says officers should have the ability to look for guns and contraband when someone is pulled over and it's likely they are armed or their immediate person or vicinity poses a threat and/or has evidence of a crime. The courts have decided that any drug suspicion means they are likely to have a weapon, which is unfortunate, but at least based in some reality.
To my knowledge, the only court (and courts are split) that has allowed electronics to be searched is when a drug dealer was pulled over in California and incident to lawful arrest, the officer downloaded the call history to help them find others involved in the drug trade.
There is NO COURT to my knowledge that would authorize warrantless search or confiscation of someone's cell data, computer data, jump drives, etc. without an arrest taking place. That is a fishing expedition and it's exactly what the Court has ruled unconstitutional under the 4th amendment.
This is where I get very frustrated with the NSAs ability to collect the call information in the first place. The only reason to collect it is to fish later—that's not how the law is supposed to work.
These are complex problems that very few judges or lawmakers have enough understanding of technologically to make informed judgement, IMHO, however.
EDIT - My best guess is that the Court will allow basic access to a cell phone taken in a lawful arrest. I.E., like the article linked above, an officer would be able to look at the call history, but not necessarily download the content of the phone. I think the offender's brief will likely ARGUE that the phone not be accessed at all, with the true hope being the Court merely limits what officers can do.
I have a hard time believing they won't let officers look at any of the electronics with them. I also hope they don't just let officers download everything carte blanche.