Several sites have noted that Apple is running low on inventory of 8 GB iPhone 3Gs. Fortune's Apple 2.0 blog notes that as of last night only 36 of Apple's 197 U.S. retail stores had stock of the 8 GB model remaining.
From this raw data several sites are speculating on possible causes of the shortage. AppleInsider has suggested that the 8 GB model may be discontinued and a 32 GB model be introduced, however there appears to be no evidence to support this claim. Another possibility includes the dedication of such models towards Best Buy outlets.
Of course another possibility remains that Apple is simply temporarily running low on supplies of the model. Without further information we regard all such possibilities as speculation.