I've been to Bolivia twice, once 20 years ago and most recently 2 years ago. during my most recent trip I climbed the normal routes on Condoriri and Illiampu and bailed from the West Face route on Potosi. The trip started in mid-June. I found the conditions to be typical early season; more snow than ice. Condoriri was in great shape. We were the first group to do Illiampu for the season and found the route to be fairly easy with the crux being 60-70 degree snow climbing in excellent condition. We bailed from the Potosi WF because of dangerous snow conditions.
I would say a little later in the season would have been better, for that year. You should contact some of the climbing agencies in La Paz and inquire about the conditions sometime in May. They'll be happy to share that with you. Based on that you can time your trip for optimized conditions.
General: Stay in La Paz area for at least 3-4 days. First day or 2 do a lot of walking the streets. Then go to Chacaltaya. as mneagle pointed out, there is no glacier but there is snow. its at 17K and very easy to get to. Great place to spend the day acclimatizing. You can also go to Zongo Pass by Potosi and hike and acclimatize. Its easy to get to.
Condoriri base is a beautiful area with a lot of opportunities to climb easy 16-17K peaks.
Logistic: Lot's of agencies in La Paz. Check out the Lonely Planet guide. It has a lot of very recent information, specially about good agencies for Potosi as it is the most popular climb in Bolivia.
Good luck and have a great time.
