I know I'm a little late to the party, but here are a few ways you could go. And for the record, there are so many ways to customize these tables so that they look exactly the way you want. Thus, IMO has the following parameters: 1) have different arguments for each cell coloring option, cell height and width, column height and width, column height and width, etc .; or 2) let the user understand something.
Here are some of the possible solutions:
library(Gmisc)
tbl <- `colnames<-`(matrix(1:9, 3, 3), c('one','two','three'))
(tmp <- htmlTable(tbl))

tbl[1,1] <- 'this is a very long cell, this is a very long cell, this is a very long cell, this is a very long cell'
(tmp <- htmlTable(tbl))

tbl[1,1] <- gsub(' ', ' ', tbl[1,1])
htmlTable(tbl)

Basically just folding any spaces and using nbsp instead
The following solution actually uses some legitimate html tags:
tbl <- `colnames<-`(matrix(1:9, 3, 3), c('one','two','three'))
tbl[1,1] <- 'this is a very long cell, this is a very long cell, this is a very long cell, this is a very long cell'
(tmp <- htmlTable(tbl))
(tmp <- gsub('<td', '<td nowrap="nowrap"; ', tmp))

The solution above replaces all cell styles (td) with one that includes nowrap. Replacing all cells may or may not be what you want, which led me to the following option: regex
tbl <- `colnames<-`(matrix(1:9, 3, 3), c('one','two','three'))
tbl[1,1] <- 'this is a very long cell, this is a very long cell, this is a very long cell, this is a very long cell'
(tmp <- htmlTable(tbl))
regmatches(tmp, gregexpr('<td.*?</td>', tmp))
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