How do I do 'dir /s /b' in PowerShell?

If you are using Powershell as a shell (and not as a script processor), you can simply type:

cmd /r dir /s /b

The /r flag tells cmd.exe to run the command and exit. In other words, you'll end at the same execution context.

For many commands, cmd /r is better than dealing with Powershell object-oriented architecture.

You can use

Get-ChildItem -Recurse | Select-Object -ExpandProperty FullName
gci -r | select -exp FullName

or

Get-ChildItem -Recurse | ForEach-Object { $_.FullName }
gci -r | % { $_.FullName }
gci -r | % FullName    # In recent PowerShell versions

(The long version is the first one and the one shortened using aliases and short parameter names is the second, if it's not obvious. In scripts I'd suggest using always the long version since it's much less likely to clash somewhere.)

Re-reading your question, if all you want to accomplish with dir /s /b is to output the full paths of the files in the current directory, then you can drop the -Recurse parameter here.

My advice to you, though: Don't use strings when you can help it. If you want to pass around files, then just take the FileInfo object you get from Get-ChildItem. The cmdlets know what to do with it. Using strings for things where objects work better just gets you into weird problems.

Adding onto Joey's answer. Starting in PowerShell 3.0, you can use the new Foreach-Object shorthand to get the FullName property.

Get-ChildItem -Recurse | Foreach-Object FullName
gci -r |% FullName

The difference is that you don't need to use curly braces ({}) or the $_ variable if all you need is a property.