It's not just digital circuits that are reaching higher and higher densities. Today you can buy power supplies in IC packages that deliver serious power as the Enpirion 3A EN5337QI demonstrates. The synchronous, buck-topology, DC-DC converter integrates an inductor and still measures only 28 mm2. The converter delivers a point-of-load power density of 149 W/in2 or 23 W/cm2 to low-voltage ICs such as processors and FPGAs.
Enpirion targets applications such as DTV, audio, computing, set-top boxes, and storage devices with the EN5337QI. The supply provides a continuous output at 3A. Input voltage can range from 2.375V to 5.5V. The design delivers 92% peak efficiency -- relying on a 5-MHz switching frequency that allows for relatively small discrete inpt and output capacitors.
Protection features integrated in the EN5337QI include over-current protection, thermal shutdown, and a under-voltage lockout circuit. The supply sells for $2.50 (1000) in a 4mm x 7mm QFN package. You can find more details on the data sheet.