Just as an update from our climb in December 2007 - thought I'd add a few words to the already excellent pages on Tajumulco on this site.
We decided to do the climb without a guide because it worked out significantly cheaper and we were coming to the end of our budget! Roughly, for two people over the two days including all bus fares and hire we paid 350 quetzales which works out at about $50 - approximately half the price of a guided tour. To get to the trailhead get the bus from the Minerva bus station to San Marcos (1.5 hours approx) then the bus from San Marcos to Tajumulco village (anything from an hour to 3 hours). Ask the bus driver to drop you off at the beginning of the path for Tajumulco - para el volcán Tajumulco.
As has been said, the great advantage with volcano climbing is that you can see the summit from the beginning and up is always a safe bet. That said, we went completely off route from the beginning and, while safe enough, it wasn't an ideal route to take. There is an obvious path across the road from the small layby where the Bus should drop you. From this point you will see the summit of the volcano (the righter of the two peaks) and a ridge leading to the col between the two peaks.The sooner you can get onto that ridge the better - the good path cuts just down to the left (as you are looking at it) side of the ridge. You will almost inevitably lose the path if you don't have a guide but that's not a problem, head for the top of the ridge (in pine forest) and you'll find a good trail. You should always be aiming for the col between the two peaks.
To reach the summit from the col, I'd recommend you traverse along under the summit on the side you came up - then cut up along the obvious path and you'll swing round to the summit point. The other way looks very easy but looked to us quite hazardous from the top.
Roughly speaking, at a reasonably relaxed pace; bus to Camp should take approx 4 - 5 hours, camp - summit 1 hour, summit - bus 3 hours.
It does get surprisingly cold in the col though (on the night we were there) nowhere near as cold as all the exaggerations we'd heard. Probably just below 0 was as low as it went. Take a decent jacket and sleeping bag. An important note is that we found absolutely no water en route so don't rely on purifying available water or snow.
We hired tents and roll mats from Icaro Tours whom I would highly recommend. We were recommended Quetzaltrekkers by all the available guidebooks and found one chap helpful but the others exceptionally unhelpful. I wouldn't recommend hiring from them. In classic Guatemalan style there are no decent maps of the route - don't worry, just get onto that pine ridge as soon as you can and you'll be fine.