Originally Posted by
Ivresse
It's probably better to wait till WotLK is near release before deciding, since there's so much missing information on a lot of professions it's hard to gauge a decision yet.
For example, with Blacksmithing we currently don't know what armor items we can place gem sockets on, or what the specialisation (Hammersmith, Armorsmith etc.) weapons and armor are going to be. Heck we don't even know if we're getting any sharpening/weightstones yet.
If you really must make a decision now though, I'd probably say Blacksmithing if you are going to be a Deathknight. Most of the Inscription items are primarily commercial (all glyphs are sellable for example), and the only self-only items that are any use to you will be the shoulder enchants, since the only other self-only items that you can make are off-hand books (useless to deathknights) and Darkmoon cards (only good for levelling standard classes).