Why can't I access the USB port on my PDA (Trimble Recon) using Serial2.dll
Why are you expecting it to
Serial2 can only communicate with Serial ports exposed as such by the OS or some other driver, like Bluetooth. As far as I know, and I have looked, there is no way to use a USB port under .NET on Windows Mobile without somebody writing a custom driver. Also the USB port on your Recon is, according to the data sheet, only a slave port so it won't be able to control USB devices even if you could get a driver written.
There may be a few others around but the only PDA I know of that has a Host USB port is the HP iPAQ214 (I have one, it has the normal slave port as well) but even then only drivers for USB mouse, keyboard and memory sticks are provided by the OS.