OK, I was wrong. I went to work this morning and looked up the information. I came home just to post a correction of my mistake. Kind of sucks when you have to have your books in two different places, plus no internet access at work.
Yes, emekthian was correct. The Tibetan letters do show tones. Here’s how:
"There are four tonal phonemes: mid (neutral), high, low, falling. These are not marked in the script, but can be deduced from the initial consonant or consonant cluster+vowel. High and low tones inhere in consonants: in a four-term series (velar, palatal, dental, etc.) the first two have high, the second two low tone: e.g. ká - khá - ga - nga; cá - chá - ja - nya.
The inherent tone of an initial can be changed by a prefixed, superscript, or subscript consonant: e.g. d prefixed to a low-tone initial raises it to high; thus ma is a low-tone consonant, but dMar /maa/, ‘red’, is high tone.
Final consonants introduce a further complication by affecting the tone, quality and length of the base vowel; e.g. na is low tone, but addition of -d, e.g. in nad /nEÈ/ produces falling tone."
—from Concise Compendium of the World’s Languages by George L. Campbell, p. 537.
There’s also a brief explanation of the spelling weirdness:
"The basic point is that prefixed letters may precede the word-initial consonant: e.g. in bsTan.’gyur, T is the initial of the first component, which is pronounced /ten/. In the classical literary language, the verb sTon /tœœn/ has a past root which is spelled bsTan, the form found in this compound, which is pronounced as a whole /ten.kuu/.
This retention of the traditional and etymological orthography means that the correspondence between sound and symbol is very weak. For example, kra, khra, gra, phra, bra, sGra, bsGra are all ways of writing the phoneme /ta/.
In Tibetan dictionaries, words are entered under word-initial consonant, i.e. ignoring the prefixed letters. Taking C=consonant, V=vowel sequence i, u, e, o, P=prefixed letter, the dictionary entry sequence is CV, CyV, CrV, PCV, PCy/rV, PPCV. Final consonants are ordered in sequence." (Ibid.)