Heat Pumps and Electric Heating - Trane heat pump froze up w/ the heat on. What gives?

i've seen heat pumps and AC units freeze up w/ the AC was on, but i've never seen one do what mine has done. i noticed that there was no air coming from the ceiling vents in my house (thus, it was getting cold and t'stat wasn't shutting the HP down). so i opened up the inside unit, and found it frozen solid (see pics).

is there any way to prevent this from happening? it's been relatively mild here the past few days (40s-50s), but at night, it gets down in the mid to upper 20s.

the weird thing is, while it's always pretty cold downstairs, the upstairs feels like it's heating just fine. i can't feel any air coming through the vents, but when the HP is running, it's pretty hot upstairs.

http://www.kyleandlara.com/hp/IMG_1723.JPG
http://www.kyleandlara.com/hp/IMG_1724.JPG

Lot of ifs here.When was freon last checked? 4 way valve outside ok? TXV in side ok or orifice there ok? Contaminate moisture dirt air in the unit. When ice is all gone, You turn heat on is big copper line hot?? Do the AUX heat strips come on?? fan inside and outside run ok,
When its back running right try 24/7 on the fan for that cold down and hot up.

ED ;)

below the coil is a washable filter that looks clogged in the pics .....that will do it

Am I missing something here? Why would the air handler side freeze up durring heat mode? I can see durring A/C mode but on heat mode wouldn't the outside coil freeze?