An FAA-appointed Designated Pilot Examiner has recently told me that it is legal to fly an airplane or glider in clouds in Class G airspace with no clearance, if the aircraft is appropriately equipped and the pilot has an instrument rating in airplanes.
On the other hand, here is a case, originally noted in an answer to a related question on ASE, where a pilot who took off from
an uncontrolled airport into clouds without a clearance or
release from air traffic control (ATC) was found to not be in violation of FAR 91.155(a), the VFR cloud clearance and visibility requirements, but was found to be in violation of FAR 91.13(a), the prohibition against operating an aircraft "in a careless or reckless manner so as to endanger the life or property of another." An interesting aspect of the pilot's defense in this case is his suggestion that by departing immediately after another aircraft which had received an ATC release and clearance, he assured himself that there would be no other IFR aircraft in the airspace above the airport 1. Other aspects of the pilot's defense were the fact that he broadcast his departure intentions on the Unicom frequency, and he monitored the ATC frequency for other traffic.
The FAA and NTSB did not find these arguments to be convincing.
There definitely seems to be a grey area here as to exactly what precautions, other than receiving an IFR clearance for adjacent controlled airspace, would satisfy the FAA as far as FAR 91.13 is concerned, in the context of operations in IMC in uncontrolled airspace. Also, one can easily imagine situations that are clearly compliant with all regulations and all accepted practices, and yet clearly create significant risks, arguably more so than the case linked to in this answer. For example, one could argue that any time any aircraft operates in IMC in uncontrolled airspace, this creates a significant risk of collision with VFR traffic which may be legally flying just outside the edges of clouds with no radio or transponder. Yet IMC in uncontrolled airspace is an accepted practice-- many airports are located in airspace that is Class G up to at least 700' AGL, yet have established instrument approaches with minimum decision heights much lower than that.
1-- Page 4 of the finding says that the respondent claimed that by following this procedure the respondent "assured himself that there would
be no other IFR aircraft in the controlled airspace above the
airport". This may be a typo-- it should perhaps read "uncontrolled" rather than "controlled"-- since there seems to be no dispute that the respondent was able to observe the prescribed VFR minimums once he reached controlled airspace.