Msquare:
There are other pages on this site, other than the forum, you know. One, quite easily to get at from the front page, "Products" show the basic specifications: 3V3. A 3.3 volt supply generated by the on-board regulator. Maximum current draw is 50 mA.
Actually the maximum current draw specification for a shield's 3.3vdc pin is a little complex due to legacy history. The current uno and meg2560 boards have a on-board 3.3vdc voltage regulator that is certainly able to supply more then 50ma of current, 100-150ma is probably possible without issue. However older arduino boards that used FTDI USB serial converter chip used the build in 3.3vdc regulator inside the FTDI chip which did carry a 50ma maximum current specification and the FTDI chip could be damage if one drew more then that amount of current from the shield 3.3vdc pin.
So what is a poor commercial shield developer to do? If they design a shield that draws more then 50ma from the attached control board's 3.3 pin then it would work for current Uno boards but might very well break older arduino boards that used the FTDI chip. That is why you will see that most commercial shield boards that require the use of 3.3vdc have their own on-shield 3.3vdc regulator, rather then draw current from the 3.3 pin.
So the Arduino folks are taking the prudent position by stating that the maximum current to be drawn from the shield 3.3 pin should be limited to 50ma or less, more for legacy compatibility then a actual technical limitation on current arduino board designs.
Lefty