Is aluminum foil porous?
There are two questions here: 


Is aluminum foil porous (particularly the types of foil sold for household cooking)?
Is foil's porosity responsible for the liquid that sometimes appears on the other side in cooking/baking applications?


I'd say the brief answers to these questions are (1) sometimes a bit, especially when not using "heavy duty" foil, and (2) perhaps responsible for a small part of the effect in some situations, though how much likely depends on the foil type/quality and how it is used.  More details below.



First, aluminum foil can be porous when it is sufficiently thin.  This is usually due to small holes left in the manufacturing process.  According to Gordon Robinson's Food Packaging: Principles and Practice (3rd ed.):


  Aluminum foil is essentially impermeable to gases and water vapor when
  it is thicker than 15 µm, but it is permeable at lower thicknesses due
  to the presence of minute pinholes.  For example 12 µm thick foil has
  a WVTR [water vapor transmission rate] of ≤0.01 g m-2 day -1, and 8-9 µm thick
  generally of 0.07-0.1 g m-2 day -1 (Lamberti and
  Escher, 2007).  These values are still far below those of most plastic
  films used for food packaging....


Other sources give slightly different numbers.  The Wiley Encyclopedia of Packaging Technology lists:


  0.001 in. (25.4 µm) and thicker is impermeable; 0.00035 in. (8.9 µm) has a WVTR of 0.02g/100in.2 (0.065m2); 24 h at
  100°F (37.8°C)


How thick is common household aluminum foil?  According to this link:


"Standard Duty" usually is between 0.0004" and 0.0007" thick.
Most foil labeled "Heavy Duty" is between .0008" and .001" thick. 
Most foil labeled "Extra Heavy Duty" is between .0011" thick and .0016" thick.


The first source above lists the minimum thickness for impermeability to be about 0.0006" (15 µm, though this may only be for gases, not liquids), while the second source says 0.001".  Other sources seem to indicate liquid impermeability requires a minimum thickness of around 0.001".  Regardless, it seems likely the "standard duty" foil has some permeability in most cases.  On the other hand, "heavy duty" foil is likely completely impermeable (or very nearly so).

But how much is this permeability?  At room temperature, it would only allow something like a hundredth of a gram through per day in a pan, probably less.  At baking temperatures, permeability would increase, but you still would likely only see a fraction of a gram of liquid getting through during a typical baking period.  And for heavy duty foil, the amount should be zero or negligible... at least theoretically.



To move on to the second question, so where does this liquid come from?  I myself have seen such liquid even with a double layer of foil or with heavy duty foil.

Other answers have suggested one likely culprit, which is condensation.  Ovens often tend to be humid places when uncovered foods are cooking, and food that is in contact with the bottom surface of a pan (and the foil) is generally still cooler than the oven (often the coolest part of the food is on the bottom), which means that humidity will condense on the surface of the pan/foil.  It's usually a lot faster for molecules (even larger ones, which often "piggyback" on evaporating water) to move this way in a hot oven than through microscopic pores in the foil.

The other likely issue is imperfections created during the handling of the foil.  A perfectly smooth layer of foil removed carefully from the roll may theoretically be impermeable, but bending, crumpling, and otherwise shaping the foil may expand or fracture some tiny pinholes (or other thin sections).  All it takes is a few tiny holes (which may not even be visible) to significantly increase the permeability.

Actual leaking may be due to poor manufacture in modern foils or to the severe crumpling done in experiments like this one. (EDIT: See discussion in this thread, which responded to that link with several other experiments.  The conclusion was that is was likely design that caused the leak, and some brands of heavy duty foil do seem impermeable to water.)  

In any case, it's clear that aluminum foil -- when used practically in home culinary applications -- can sometimes allow some liquid to flow through at a greater rate than the theoretical porosity would suggest.