Can a commerical jet fly into a tropical cyclone?
If the wind shear (sometimes called wind gradient, which in my opinion is the better term) encountered in that part of the cyclone the aircraft is penetrating is less than what the aircraft can handle, then, yes, an aircraft can safely fly into a cyclone.

What the aircraft can handle varies greatly depending on the speed at which it is being flown. Large aircraft have a recommended turbulence penetration speed, both in terms of knots and mach. In the absence of such a recommendation, V speeds that are relevant are Va, Vb, and Vo. http://en.wikipedia.org/wiki/V_speeds is a good V speed terminology reference.

I do not know what typical wind gradient values within a cyclone are or what their distribution within the cyclonic area is. However, I would be surprised if they were significantly greater than gradients within tropical thunderstorms, and aircraft do penetrate such.

In season there's a roughly north-south line of thunderstorms that regularly develops between Sumatra and Sri Lanka with tops greater than 50,000 feet. Lacking the fuel to go around the line, we would regularly go through it using radar to avoid the worst. However, with those old radars (back in the 1990s in 747s), it was not uncommon for us to wind up in a cell.