See this post from Dowd's Spirits Notebook:
It's not the same as last year's Knob Creek shortage, when the bourbon distiller reported it was temporarily out of enough aged whiskey to bottle.
In this instance, the folks who make Angostura Bitters apparently have plenty of product, What they don't have is enough bottles to put it in.
The globally popular cocktail ingredient has been in short supply since the second half of 2009 because of an ongoing dispute between the House of Angostura in Trinidad and Tobago and the company that supplies its bottles. Production was halted in November, although since then a limited number of bottles have been delivered.
Some bars and restaurants have been hoarding their supply until the pipeline reopens. At the moment, the manufacturer is being vague about when that may be.