The lord chief justice in the UK has made a clear statement that tweeting in the court room is ok for journalists, even though the general rule of thumb is that mobile phones must be turned off.
"The normal, indeed almost invariable, rule has been that mobile phones must be turned off in court. There is however no statutory prohibition on the use of live text-based communications in open court," said the guidance released by the lord chief justice.
There is a caveat that this only applies to journalists and not to individuals who will still be required to turn off their mobile phones when court is in session. If you absolutely must tweet during proceedings, you'll need to make a special request to the judge which seems unlikely to be permitted unless the circumstances are extreme.
The guidance continues, with point 10 specifically adressing the issue in hand. "It is presumed that a representative of the media or a legal commentator using live, text-based communications from court does not pose a danger of interference to the proper administration of justice in the individual case. This is because the most obvious purpose of permitting the use of live, text-based communications would be to enable the media to produce fair and accurate reports of the proceedings. As such, a representative of the media or a legal commentator who wishes to use live, text-based communications from court may do so without making an application to the court."
The document still leaves the ultimate choice of whether to allow it with the presiding judge, but in general it seems that it'll be permissable. The only real clause is that you must do your tweeting quietly.
"Subject to these considerations, the use of an unobtrusive, hand held, silent piece of modern equipment for the purposes of simultaneous reporting of proceedings to the outside world as they unfold in court is generally unlikely to interfere with the proper administration of justice."