Packed fonts allow you to use the "Pack chars in multiple channels" feature in BMFont. When this is enabled, each channel of your texture (4 channels in a 32 bit image) stores an independent set of glyphs. This means that you can have 4x the number of glyphs in a single texture - a 2048x2048 texture can store the equivalent amount of data as a 4096x4096 texture.
Export your font from BMFont, ensuring you have Bit Depth set to 32, and Pack chars in multiple channels is ticked in "Export Options".
Set-up this font as you would normally - you can't currently import the font into a sprite collection.
Switch the shader on the material to tk2d/Goodies/PackedTextMesh.
Use the font as you would normally, but keep in mind the limitations listed below.