Maciej,
If you put 10dB or 15dB in-line attenuators in the Rx ports of the WS-G5487 1000BASE-ZX GBIC, and you use a single-mode fiber patch cable at least 2 meters in length, you will be fine. This is often done in a lab setup, or to test a configuration prior to installing systems kilometers and kilometers apart.
Previous post by RBergman is correct in pointing out that for an installation under 10km, it is hard to cost-justify installing ZX GBICs where LX/LH GBICs will get the job done for less than 15% of the price. However, there are circumstances where the ZX may be the solution in a run under 10km. For example, if you have to use a single-mode fiber span with end-to-end measurable loss greater than the loss budget of the LX/LH, but less than the loss budget of the ZX (9.5dB or 10.5dB for the LX/LH, vs. 23dB or 24dB for the ZX, depending on which Cisco specs you read). I had a situation once where it was less expensive and quicker to get a customer up and running using a pair of ZX GBICs than it would have been to reinstall several km of new single mode fiber. Besides the larger loss budget we got, we also lost less in per-kilometer attenuation at the 1550nm vs. 1300nm wavelengths.
In any short-distance situation with ZX GBICs, the key is to PROTECT THE RECEIVE OPTICS. Cisco specifications say the ZX's maximum Tx optical power is +4.77dBm, +5dBm, or +5.2dBm, depending on what you read. The maximum Rx is -3dBm or 0dBm, again depending on what you read. So, using the worst-case extremes, +5.2dBm transmitted directly into a receive optic capable of at most -3dBm results in an overdrive of 8.2dB. If you protect the Rx optics by giving them 9dB or greater "sunglasses", then as long as you don't overattenuate the signal to the point of exceeding the loss budget, you are guaranteed to work.
Hope this helps.
