Deliberate misspelling is a common spam technique which most spam filters account for. I imagine that when filters encounter an unfamiliar word (not in the database), they cycle through all of its possible forms given the possible forms of each letter and check that none of the results match a familiar word before confirming it as a new word and adding it.
And the spamming technique of having an innocent subject line, possibly some innocent text and the spam message following that, is insidious for two reasons. The first is that having examined the legit looking subject line, people will often open the e-mail based on that. The second is Bayesian poisoning which fools some spam filters - spammers will often insert legit looking paragraphs which contain non-spammy words and then follow those with the spam content. Since Bayesian spam filters tend to consider the spamminess of all words in the e-mail, the legit paragraphs lower the overall spamminess of the e-mail and can allow this spam to slip through the filter. There are simple ways of preventing common Bayesian poisoning though.
A very common spamming technique I see nowadays is image spam. The idea there is that the spam is contained within the image so text-based spam filters cannot process it. But Gmail uses optical character recognition (as used in scanners) to extract text from those pictures - and it probably treats the text quite suspiciously.
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