A quick warning - as of this month (november 2007) you now CANNOT obtain an Eritrean tourist visa in Sanaa, Yemen unless you have a Yemani residency permit. It has just changed (..and I have just bought a non-refundable air ticket!!) and as of yet I cant find a way around it. Obtain the visa before you leave home. This obviously makes guide book information obsolete and should be double-checked.