There are a couple of ways: audio tweeting and dialup.
Speak2Tweet: Speak2Tweet is an "audio cast autotweeter." To access Speak2Tweet from "inside Egypt. Call +16504194196 or +390662207294 or +97316199855 to leave a tweet and hear tweets."
Dialup: a number of people and groups are providing dialup numbers for those inside Egypt trying to get out to Tweet, email and so on.
If you have additional methods, please let us know in the comments.
Thanks to Carl Levinson