At RF and microwave frequencies, the more pertinent question often is, "What is the package?" At higher frequencies, the package can have as much effect on the performance as the circuit it houses and it is better to think of the package and the device as inseparable. Years ago, custom packages were needed to ensure that the performance of microwave circuits was not "thrown away" once in a package. For applications where protecting the circuit was tantamount to success, such as in aerospace and military systems, package design specialists such as StratEdge  built strong reputations with high-reliability metal-ceramic packages for use past 40 GHz. The company continues to custom design packages that, while not inexpensive, provide the highest performance levels possible at microwave frequencies.
Of course, cost pressures have forced many device and circuit designers to seek more standard, lower-cost packaging solutions. Companies such as Mini-Circuits  have long balanced the tradeoffs between cost and erformance in drop-in and surface-mount RF/microwave packaging. The firm has kept pricing reasonable by choosing packages that are low in cost but don't compromise performance.
The ideal package would be electrically "invisible" with lossless interconnections between the circuit within and the outside world. It would also afford protection against contamination, moisture, and other environmental factors. The interconnections would be practical, with ground connections and electrical paths when mounted with a variety of automatic assembly equipment or even by hand.
There is no "ideal" package, of course, and few come close. But circuit and component designers have more packaging options than ever, with excellent high-frequency performance. Most of the newer standard packages are developed for small footprints and standard die sizes, encouraging IC and device designers to follow standard foundry rules for pad placements and other circuit considerations. Such factors also simplify the use of standard wafer probes and test equipment when it is time to characterize die before packaging.
Perhaps one of the major boons to device and IC designers faced with evaluating package options is the increasing number of package models in high-frequency design programs, such as the Advanced Design System (ADS) from Agilent Technologies  and Microwave Office from AWR . These software tools essentially allow designers to "package" their circuits in the computer before trying the real thing, helping to tame those difficult tradeoffs when finding a housing for that "perfect" circuit.