Effect of heat on CAT5 Ethernet Cabling
The spec says -55C to +60C so you may be exceeding that rating.
http://en.wikipedia.org/wiki/Category_5_cable#Electrical_characteristics_for_Cat.5e_UTP
Personally I think it will be ok if you lag the cable and possibly by a good quality Cat5e or Cat6 cable.
Personally I wouldn't run a cable like that without some kind of insulation/heat shield, and I would only do that after looking for alternate ways of routing the cable.
Problems I see are:
Electricity doesn't work well with heat (heat negatively affects conductivity) - Dave Cheney already pointed out the specs regarding acceptable temperates for Cat5e.
The cables plastic jacket is likely to melt over time (or rather it will soften up just enough that it begins to fray apart under it's own weight), exposing the wires inside. This will likely result in packet loss over the cable.
And most importantly, you could start a fire. Either from the plastic jacket heating up enough that it catches fire (burning wires are great - it spreads through walls, and releases toxic gases) or after the cable frays, the exposed wire could touch the metal and short, resulting in an electrical fire (the fact that there is water involved isn't great either).
Don't take all this as overly dismissive - I've personally taped RJ12 to the outside of building ducts as a quick way to route in an extra phone - however an old style cast iron system does not sound good. A simple test would be, with the heating jacked up, can you hold onto the pipe for long without getting a burn?
According to the table for fire forensics* (reference below), plastics melt in the temperature range of a low-pressure steam heating pipe. From the online / book research I've done pipes and radiators max out at around 120°C, and are typically closer to boiling point (100°C). **
Forced hot water operate at lower range, typically 60°C-82°C (140°F-180°F) depending on the load on the system. If your needs do not exceed operating at 140°F (60°C) regular network cable might be fine. There is also industrial high temperature cat5 cable that operate at 75°C even 80°C. I consulted globalspec.com and cable data sheets. From what I've found there is not an easy universal way to find the operational temperature from looking at the cable. You might be able to use the UL listing number or standards codes if these are printed on your cable, or information on the packaging or receipt to find the data sheet for the cable you have.
You could insulate your cable, but insulating the heating pipe will offer more protection and also save energy.
According the same fire forensics table the lowest ignition temperature for plastics is 349°C, far above any of these operational temperatures for water or steam heating systems so there is no fire hazard whatsoever from running the network cable near heating pipes. Fabrics have a much lower ignition temperature, and even these temperatures are far above the operational temperatures for steam and forced hot water home heating systems.
Electric baseboards are another matter. Do not network too close to them. Also, don't network next to your dryer duct or for that matter your boiler flu pipe. Dryer exhaust pipes, especially from a gas heated dryer, can be very hot - much hotter than pipes for forced water or steam heat. Blocked dryer vents are one of the most common causes of house fires, so clean yours out regularly.
While I believe this all to be factual and sensible, I am a software developer, not a fire marshal or building inspector. This should not be taken as professional advise and comes with no guaranty what so ever. Proceed at your own risk.
*Table data source: http://www.tcforensic.com.au/docs/article10.html#2.1 Operational temperatures for heating systems were taken from a variety of sites about heating system maintenance and fire safety. Please verify the numbers for your system by taking some measurements.
** High-pressure steam systems in industrial, and commercial settings operate at considerably higher temperatures than home heating systems.