I would say get the flash. It will allow you more flexibility in the long run. IMHO the kit lens is fine for most work, it is a general purpose lens. Read strobist and then think again.
I was/am in the same situation as you and went with the flash so i could start to play with the light using the CLS system on the D90.
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D90 SB 900/SB 600
DX AF-S 35 mm 1:1.8 G
DX AF-S 18-105 mm1:3.5-5.6 G ED
AF-S 70-300 mm 1:4.5-5.6 G
AF-S Micro 105 1:2.8 G ED
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