This arose out of a comments discussion in this question.
The only airspace I really know is the UK. Here, a controller may issue a clearance to exceed the 250kt limit below 10,000ft for reasons other than minimum safe flying speed, e.g. to expedite a transition for spacing.
Is this common, e.g. in the US? I know it's open ended but I'm interested in whether this is just some "local" thing.
Not a local thing at all!
In Australia, it is extremely common to hear, especially for aircraft on departure. Listening to LiveATC, particularly Sydney International Approach/Departure (YSSY) you will quite often hear ATC give an aircraft permission to exceed 250KIAS below 10,000ft.
Qantas 123: Approach, QFA123, with you passing 2,300, climbing 5,000..
Sydney Approach: QFA123, Identified, Climb FL240, cancel speed restrictions below 10,000..
There are a number of reasons this is used. The first, I have already hinted to in the example radio call above.
Departures: It is extremely common to hear ATC cancel speed restrictions on departing aircraft. This gives better separation for aircraft departing in the same direction. For example, one of the busiest sectors in the world is Sydney - Melbourne Australia. The common SID's out of Sydney for Melbourne are the DEENA4 WOL transition and the WOL1 Departure, depending on operational runway. As a large amount of aircraft use these departures, the further the lead aircraft can get on the SID prior to the trailing aircraft becoming airborne, the better.
Arrivals: Again, using Sydney, they receive a large amount of Regional Turbo-Prop traffic (Dash 8-Q300/Q400, Saab 340B, BE350). You will commonly hear Approach cancel speed restrictions on these aircraft on decent so they can give them more space on the approach. They land reasonably slower than a B747/A380, so the more room, the better.
Another reason is simple. These guys are operating commercial operations, hence faster is better, especially on descent. So if there is no or low traffic in the TMA, ATC will commonly release the speed restriction on aircraft to accommodate the company operations.
In the US, the 250 kt speed limit is codified in 14 CFR 91.117
§91.117 Aircraft speed.
(a) Unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots (288 m.p.h.).
(b) Unless otherwise authorized or required by ATC, no person may operate an aircraft at or below 2,500 feet above the surface within 4 nautical miles of the primary airport of a Class C or Class D airspace area at an indicated airspeed of more than 200 knots (230 mph.). This paragraph (b) does not apply to any operations within a Class B airspace area. Such operations shall comply with paragraph (a) of this section.
(c) No person may operate an aircraft in the airspace underlying a Class B airspace area designated for an airport or in a VFR corridor designated through such a Class B airspace area, at an indicated airspeed of more than 200 knots (230 mph).
(d) If the minimum safe airspeed for any particular operation is greater than the maximum speed prescribed in this section, the aircraft may be operated at that minimum speed.
Note in section (a) that only the administrator may authorize operations below 10,000 feet MSL at speeds above 250 KIAS. The only exception to this is part (d) which applies to aircraft that cannot safely operate below this speed. As far as I know this only applies to specific heavy airplanes. ATC does not have the authority to issue instructions contrary to regulation.
Historically the FAA has experimented with the ability of controllers to cancel the 250 kt speed limit within certain class B airspace, notably in the Houston, TX class B. That program was discontinued and currently no controller may grant you an exception to the regulation.
Note that the 200 kt speed limit in class C and D surface areas and any speed restriction published on a chart (departure and arrival procedures) can be cancelled by ATC.
In the US, speed restrictions are frequently cancelled, either to ensure or to increase separation between aircraft, as per ATC Handbook 4/3/14 JO 7110.65V, Section 5-7. Speed Adjustment:
5−7−1. APPLICATION Keep speed adjustments to the minimum necessary to achieve or maintain required or desired spacing.