1. Capsules for inhalation which contain as the inhalable powder tiotropium in admixture with a physiologically acceptable excipient, characterised in that the capsule material has a reduced moisture content (TEWS or halogen drier moisture content of less than 15%). 2. Capsules for inhalation according to claim 1, characterised in that the capsule material is selected from among gelatine, cellulose derivatives, starch, starch derivatives, chitosan and synthetic plastics. 3. Capsules for inhalation according to claim 2, characterised in that the capsule material used is gelatine in admixture with other additives selected from among polyethyleneglycol (PEG), preferably PEG 3350, glycerol, sorbitol, propyleneglycol, PEO-PPO block copolymers and other polyalcohols and polyethers. 4. Capsules for inhalation according to claim 3, characterised in that the capsule material contains, in addition to gelatine, PEG in an amount of 1-10% (wt.-%), preferably 3-8 %. 5. Capsules for inhalation according to claim 3 or 4, characterised in that the capsule material has a TEWS or halogen drier moisture content of less than 12%, particularly preferably <= 10%. 6. Capsules for inhalation according to claim 2, characterised in that the capsule material is selected from the cellulose derivatives hydroxypropylmethylcellulose, hydroxypropylcellulose, methylcellulose, hydroxymethylcellulose and hydroxyethylcellulose. 7. Capsules for inhalation according to claim 6, characterised in that the capsule material has a TEWS or halogen drier moisture content of less than 8%, particularly preferably <= 5%. 8. Capsules for inhalation according to claim 2, characterised in that the capsule material is selected from the synthetic plastics polyethylene, polycarbonate, polyester, polypropylene and polyethylene terephthalate. 9. Capsules for inhalation according to claim 8, characterised in that the capsule material is selected from polyethylene, polycarbonate and polyethylene terephthalate. 10. Capsules for inhalation according to claim 8 or 9, characterised in that the capsule material has a TEWS or halogen drier moisture content of less than 3%, particularly preferably <= 1 %. 11. Capsules for inhalation according to one of claims 1 to 10, characterised in that the inhalable powder contains 0.001 to 2% tiotropium in admixture with a physiologically acceptable excipient. 12. Capsules for inhalation according to claim 11, characterised in that the excipient consists of a mixture of coarser excipient with an average particle size of 15 to 80 mum and finer excipient with an average particle size of 1 to 9 mum, the proportion of finer excipient in the total quantity of excipient being 1 to 20%. 13. Capsules for inhalation according to claim 12, characterised in that the tiotropium is in the form of its chloride, bromide, iodide, methanesulphonate, para-toluenesulphonate or methylsulphate. 14. Use of capsules for inhalation according to one of claims 1 to 13 for use in an inhaler which is suitable for administering inhalable powders. 15. Use according to claim 14 for the treatment of asthma or COPD. 16. Use of empty capsules which are characterised by a TEWS or halogen drier moisture content of less than 15%, for preparing tiotropium-containing capsules for inhalation according to one of claims 1 to 13.