What are the current and voltage limits when determining low vs. high voltage wiring?
National Electrical Code (Article 250.20 section A) defines "Low Voltage" as it applies to home distribution systems as less than 50 volts.


  (A) AC Systems of Less Than 50 V. Alternating-current systems that
  operate at less than 50 V are not required to be grounded unless:
      (1) The primary exceeds 150 V to ground
      (2) The primary is ungrounded


This includes systems such as cable television, network and telephone, doorbell, etc. Note that standard telephone wiring can typically "ring" at 48 volts and still provide an electric shock.

The reason the code makes the distinction for low voltage is so that some types of wiring can be exempt from additional safety requirements such as conduit usage, breakers, installation method, etc. Even so, there are still violations where installers confuse the issue (Top ten violations in low voltage systems PDF).

There are (to my knowledge) no current limits for data lines, except for the rating of the cable itself. Since most UTP network and telco cables use 24 or 26 gauge wire, this would theoretically be around 3.5 amperes, but you have to consider length, temperature, etc. to properly determine current load (discussion at allaboutcircuits).

EDIT

Guessing that you might be wanting to use data lines for higher power applications, I thought I'd add some additional commentary. If you are wanting to use Cat5 for power-over-ethernet applications, you'll probably want to stay under 1A or less per conductor. This is because of the voltage drop and resistance on longer distances. Keep in mind that higher voltages are more suitable for transmission (longer distances) as long as you stay under the 50V "low voltage" specification. Even though charts show that 24AWG can carry 3.5A, I wouldn't consider this safe unless working with a very short cable. Power-over-ethernet standards are confusing, but generally they are 48 volts or less at 600mA or less.

Electrical characteristics (at Wikipedia) for UTP Cat5E show a max current of 0.577A per conductor and a max voltage of 125V.

EDIT 2:

In case the linked list of top ten low-voltage violations dies again, here they are in a nutshell:


Audio cables run in cable trays with other low-voltage cable. [725.56(F)]
Power cords run above ceilings. [400.8(5)]
Cables suspended from a cable tray system. [392.6(J)]
Unlisted broadband backbone cables run inside the building for more than 15 m (50 ft). [820.113 exception 2]
Secondary protection used in lieu of primary protection. [800.90(D)]
Patch cords run in plenum spaces. [800.154(A)]
Low-voltage cables run from multiple output power supplies with outputs not classified as Class 2 installed in the same raceway or cable tray with other low-voltage cables. [725.55(A)]
Ungrounded or improperly grounded primary or secondary protectors. [800.170(A) and (B)]
Unlisted communications cable coming into the building in EMT conduit and extending beyond 15 m (50 ft). [800.2]
Insulated grounding backbone cables run in air-handling spaces in the cable tray. [800.154]