In a word, no. Card counting is not illegal.
You can not use any devices to assist you in card counting. Pocket sized computers and other electronic devices that can be used as card counting aides are illegal. If you are using such a device you will be arrested in Las Vegas. A card counter can only use his own intellect to card cards.
A casino will be on guard for card counters. They watch for players who play in a business like manner. They look for players who do not interact with the dealer or other patrons. They look for players using basic strategy. Casinos will count along with a suspect card counter. A player will be labeled as a counter when the casino sees the player raise his bet when the count is in his favor.
Team play was big in the 1990s. A team would consist of one or more players at a table betting only the table minimum and counting cards. Since these players never increased their wager, the casinos could not spot them as counters. These players then signal another, "big money" player. The "big money" player wanders from table only stopping where the count is in his favor and only betting large amounts. Without the typical wager increases of a single card counter, teams of players were difficult for the casino to spot.
A casino may escort you off promises, read you the riot act, and threaten you with trespassing should you return. They may share your picture and profile with other casinos. However, this is not likely unless you are wagering large sums of money.
A card counter who is counting poor or who is losing is likely to be allowed to play. A casino is happy to have anyone lose at any of their games. A card counter who is winning will often provoke some action by the casino. The easiest thing for the casino to do is instruct the dealer to shuffle more frequently, sometimes every third or fifth hand. This makes the counter constant reset his count and he will never see the positive count he needs.
In order to prevent being identified as a card counter, a player can do several things. First, the player should act in a relaxed manner. Second, it may be prudent to break basic strategy. For example, you could split tens every once in a while. A casino is likely to label you a recreational player if you break basic strategy now and then. Finally, a player can be careful with his wager. A five to ten times increase in the wager is likely to be ignored. Beyond ten times increase will draw scrutiny. Also, some players recommend changing your wager in proportion to the running count. Thus the wager is always changing and it's hard to notice increases.