Bug was fixed in 3.5.6

Edit! I've changed the way I've done my workaround, so maybe it'll be more effective.

I keep a reference in one of my classes to an instance of TouchScreenKeyboard:

private TouchScreenKeyboard keyboard = null;

in my Start(), I instanciate a keyboard:

keyboard = new TouchScreenKeyboard("", TouchScreenKeyboardType.Default, true, true, false, false, "");
keyboard.active = false;

then, when I want to ensure my keyboard is re-shown, I do:

keyboard.active = false;
TouchScreenKeyboard.Open("current text from text area", TouchScreenKeyboardType.Default, true, true, false, false, "");

I call that code when the user taps on an area that would be a text area. Something like this (pseudocode)

foreach (textarea t in alltextareas)
    if (Input.GetMouseDown(0) && t.MouseIsOver && t.IsClickable)
        ForceShowKeyboard()

Surely there's a better way, but I imagine it will require Unity to change how they interact with the keyboard API on ICS.

Good luck!